THE  ELECTRIC  FURNACE 


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Frontispiece 


THE  ELECTRIC  FURNACE 


ITS  CONSTRUCTION,  OPERATION 
AND  USES 


BY 

ALFRED  STANSFIELD,  D.Sc. 

ASSOCIATE   OF  THE   ROYAL  SCHOOL  OF  MINES;    FELLOW   OF 

THE   ROYAL  SOCIETY   OF  CANADA;   BIRKS   PROFESSOR 

OF   METALLURGY   IN   MCOILL   UNIVERSITY 

MONTREAL 


SECOND  EDITION 
SECOND  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC, 

239  WEST  39TH  STREET.    NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 

6  &  8  BOUVERIE  ST.,  E.G. 

1914 


3ft 


COPYRIGHT,  1914,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


TUB . MAPLE. PKES8. YORK • PA 


DEDICATED   TO 

EUGENE  E.  R.  HAANEL,  PH.D.,  F.R.S.C. 

DIRECTOR   OF   MINES,   OTTAWA 

IN   RECOGNITION   OF   THE   SERVICE   HE   HAS   RENDERED 
TO   ELECTRIC   SMELTING   THROUGHOUT  THE   WORLD 


362500 


PREFACE  TO  SECOND  EDITION 

Since  the  first  appearance  of  this  book,  in  1907,  the  development 
of  the  electric  furnace  and  its  uses  has  been  so  rapid  that  this  edition 
has  been  increased  to  more  than  twice  the  size  of  the  first,  and  the 
whole  has  been  reset. 

Care  has  been  taken  to  include,  as  far  as  possible,  all  recent  devel- 
opments of  importance,  but  as  the  preparation  of  this  edition  has 
occupied  at  least  three  years,  it  has  been  difficult  to  bring  each  part 
as  closely  up  to  date  as  is  desirable  in  so  up  to  the  minute  a  subject 
as  electric  smelting. 

I  wish  to  express  my  indebtedness  to  the  following  gentlemen  and 
others  who  have  helped  me  with  information,  advice,  or  the  use  of 
illustrations  for  the  present  edition:  The  Canadian  Boving  Com- 
pany, The  Carborundum  Company,  Electro  Metals  Company,  Mr. 
J.  W.  Evans,  Mr.  A.  M.  Fairlie,  Dr.  K.  G.  Frank,  Mr.  J.  H.  Gray, 
Dr.  Eugene  Haanel,  Prof.  L.  A.  Herdt,  Dr.  Carl  Hering,  Dr.  R.  S. 
Hutton,  Messrs.  Harbison- Walker,  Messrs.  Leavitt  and  Company, 
Mr.  F.  Louvrier,  Mr.  Dorsey  A.  Lyon,  The  Norton  Company,  Dr. 
H.  N.  Potter,  Prof.  J.  W.  Richards,  Mr.  T.  D.  Robertson,  Mr.  E.  R. 
Taylor,  Titanium  Alloy  Manufacturing  Company,  Mr.  F.  J.  Tone, 
Mr.  R.  Turnbull,  Mr.  W.  R.  Walker,  Mr.  T.  L.  Willson  and  Mr. 
R.  A.  Witherspoon. 

I  am  very  greatly  indebted  to  my  wife,  who  read  the  whole  book 
with  me  in  proof,  and  to  Mr.  J.  W.  Hayward,  who  drew  nearly  all 
the  new  illustrations  for  this  edition,  and  read  a  large  part  of  the 
manuscript. 

ALFRED  STANSFIELD. 
MONTREAL, 
December  i,  1913. 


vii 


PREFACE  TO  FIRST  EDITION 

On  my  first  visit  to  Canada,  in  1897,  I  constructed  an  electric 
furnace  and  showed  it  in  operation  at  a  lecture  on  Canada's  metals, 
which  was  delivered  by  the  late  Sir  William  Roberts-Austen.  The 
application  of  electrical  heat  to  Metallurgy  has  always  interested 
me  greatly  and  I  hope  that  this  little  book  may  serve  to  instil  this 
interest  in  others,  and  to  help  forward  the  application  of  electric 
smelting  in  a  country  which  is  so  rich  in  water-powers  and  mineral 
resources. 

This  book  originated  in  a  series  of  papers,  written  about  a  year 
ago  for  the  "Canadian  Engineer,"  in  which  I  endeavored  to  present, 
as  simply  as  possible,  the  principles  on  which  the  construction  and 
use  of  the  electric  furnace  depend,  and  to  give  an  account  of  its 
history  and  present  development. 

The  original  papers  were  written  at  a  time  when  the  experiments 
of  Dr.  Haanel,  at  Sault  Ste.  Marie,  were  attracting  public  attention, 
and  a  large  section  of  the  book  has  been  devoted  to  the  consideration 
of  these  and  other  advances  in  the  electrometallurgy  of  iron  and 
steel. 

I  wish  to  thank  all  who  have  helped  me  in  the  preparation  of  this 
book,  including  Dr.  Haanel,  whose  valuable  monographs  have 
formed  the  basis  of  my  chapter  on  iron  and  steel,  and  to  whom  I 
am  indebted  for  additional  information  on  this  branch  of  the  sub- 
ject: Prof.  J.  W.  Richards,  who  has  taken  an  interest  in  my  work, 
and  whose  book  on  "Metallurgical  Calculations"  has  been  of  con- 
siderable assistance  in  writing  the  chapter  on  furnace  efficiencies; 
Mr.  E.  A.  Colby,  who  gave  me  information  in  regard  to  his  induction 
steel  furnace  and  a  sketch  for  Fig.  25;  Mr.  Francis  A.  J.  Fitzgerald, 
who  supplied  me  with  the  data  for  Table  X.;  the  editor  of  the 
"Electrochemical  and  Metallurgical  Industry,"  who  loaned  the 
block  for  the  frontispiece,  and  the  International  Acheson  Graphite 
Company,  who  gave  me  information  about  their  furnaces  and  lent 
the  block  for  Fig.  40.  I  also  wish  to  thank  those  of  my  personal 
friends  who  assisted  me  in  the  tedious  work  of  proof-reading. 

ALFRED  STANSFIELD. 

McGiLL  UNIVERSITY,  MONTREAL, 
November,  1907. 

viii 


CONTENTS 

PAGE 
PREFACE    v 

CHAPTER  I 

HISTORY  OF  THE  ELECTRIC  FURNACE 1-16 

The  electric  arc — Early  electric  furnaces — W.  Siemens'  furnaces — 
Cowles  Brothers'  furnaces — Aluminium  furnaces — Moissan's  re- 
searches— Production  of  the  Diamond — Willson's  carbide  furnace 
• — Carborundum — Nitrates  from  the  air — Ferro-alloys — Iron  and 
steel — Zinc. 

CHAPTER  II 

DESCRIPTION  AND  CLASSIFICATION  OF  ELECTRIC  FURNACES 17-38 

Definition — Heat  produced  by  electric  current — Essential  parts  of 
electric  furnace — Classification — Arc  furnaces — Resistance  fur- 
naces— With  special  resistor — Without  special  resistor — Electro- 
lytic furnaces — Chart  of  classification. 

CHAPTER  III 

EFFICIENCY  OF  ELECTRIC  AND  OTHER  FURNACES  AND  RELATIVE  COST 

OF  ELECTRICAL  AND  FUEL  HEAT 39~S4 

Cost  of  electrical  energy — Efficiency  of  furnaces — Calculation  of 
furnace  efficiencies — Heat  units — Rate  of  heating — Temperature 
and  heat  of  melting  metals — Calorific  power  of  fuel — Table  of 
calorific  powers — Calculation  of  efficiency  of  electric  steel  furnace — 
Cost  of  electrical  energy. 

CHAPTER  IV 

CONSTRUCTION  AND  DESIGN 55-117 

Introduction — Materials  of  furnace  construction — Fire-clay  bricks 
— Silica  bricks — Silica  sand — Lime — Canister — Magnesia — Dolo- 
mite— Chromite — Bauxite — Alundum — Carbon — Carborundum — 
Crystolon — Silundum — Siloxicon — Table  of  refractory  materials — 
Thermal  conductivity  of  furnace  materials — Table  of  thermal  con- 
ductivity and  resistivity — Radiation  and  convection  of  heat — 
Table  of  contact  resistivity — Furnace  for  testing  heat  losses — Cal- 
culation of  heat  losses — Furnace  with  external  gas-heating — Fur- 
nace walls  without  refractory  materials — Resistors — Table  of 
electrical  resistivity — Electrical  resistivity  of  heated  fire-bricks — 
Electrodes — Dimensions  of  electrodes — Laws  of  electrode  heat 
losses — Determination  of  electrode  heat-losses — Table  of  proper- 
ties of  electrode  materials — Electrode  holders. 

ix 


x  CONTENTS 

CHAPTER  V 

PAGE 

OPERATION  OF  ELECTRIC  FURNACES 118-149 

Electrical  supply — Alternating  current — Transformer  with  voltage 
regulation — Direct  current — Polyphase  currents — Electric  power 
— Electric  measurements — Rate  of  heat-production  in  electric 
furnaces — Voltage  of  electric  furnaces — Of  arc  furnaces — Of 
resistance  furnaces — Current  density  in  furnaces — Pinch  effect — 
Regulation  of  electric  furnaces — Measurement  of  furnace  tempera- 
tures— Resistance  pyrometer — Thermo-electric  pyrometers — Op- 
tical pyrometers — Scale  of  temperatures. 

CHAPTER  VI 

LABORATORY  FURNACES 150-172 

Testing  furnaces — Arc  furnaces — Button's  pressure  furnace — Re- 
sistance furnaces — Furnaces  with  metallic  heating-coils — Howe's 
crucible  furnace — Furnaces  with  carbon  resistors — Lampen's  tube 
furnace — Crucible  furnaces — Arsem  vacuum  furnace — Resistor  tube 
furnace — Barker  furnace — Smelting  furnaces — Heroult  steel  furnace 
— McGill  steel  furnace — Colby  steel  furnace — Pig-iron  furnace — 
Adjustable  electrode  holder — Silicon  furnace — Power  for  electric 
furnaces  at  McGill. 

CHAPTER  VII 

THE  PRODUCTION  OF  PIG  IRON  IN  THE  ELECTRIC  FURNACE 173-211 

Varieties  of  iron  and  steel — Production  of  pig  iron — Electrical  pro- 
duction of  pig  iron — Electric  furnaces  for  iron  smelting,  Heroult — 
Keller— Harmet—Haanel-HerouU-^—Turnbull-Heroult— Possibilities 
in  electric  smelting — Ideal  furnace — Recent  developments  in 
electric  reduction  furnaces — Domnarfvet — Frick — Californian — 
Helfenstein — Trollhattan — Electric  furnace  design. 

CHAPTER  VIII 

THE  PRODUCTION  OF  STEEL  FROM  METALLIC  INGREDIENTS 212-249 

Introduction — Electrical  production  of  steel — Series-arc  furnaces — 
H&roult — i$-ton  Heroult — Keller — Single-arc  furnaces — Girod — 
Keller — "  Electro-metals  " — Induction  furnaces — Kjellin — Colby — 
Grb'nwall — Rochling-Rodenhauser — Frick — Resistance  furnaces — 
Gin — Bering. 

CHAPTER  IX 

THE  PRODUCTION  OF  STEEL  FROM  IRON  ORE 250-264 

Electric  steel  smelting — Stassano's  furnace — Experiments  of  Brown 
and  Lathe — Evans'  experiments — Evans-Stansfield  furnace — 
Keeney  furnace — Summary  of  electric  steel  smelting. 


CONTENTS  xi 

CHAPTER  X 

PAGE 

THE  FERRO  ALLOYS  AND  SILICON 265-281 

Ferro  alloys — Analyses  of  ferro  alloys — Ferro  silicon — Silicon. 

CHAPTER  XI 

GRAPHITE  AND  CARBIDES 282-308 

Graphite — Production  of  graphite  in  electric  furnace — Acheson's 
graphite  furnace — Electrode  furnace — Graphitized  electrodes — 
Unctuous  graphite — Carborundum — Improved  carborundum  fur- 
nace— Silundum — Siloxicon — Calcium  carbide — Ingot  furnaces — 
Willson — Improved  Willson—Bullier — Horry — Bradley — Tapping 
furnaces — Helfenstein — Resistance  furnaces — Production  of  cal- 
cium carbide — Uses. 

CHAPTER  XII 

THE  ELECTRIC  SMELTING  OF  ZINC  AND  OTHER  METALS 3QQ-345 

Zinc  smelting — Electric  zinc  smelting — Cowles — Johnson — De  Laval 
Salgues — Stansfield — Snyder — Energy  needed  for  zinc  smelting — 
Zinc  smelting  at  McGill — Production  of  liquid  zinc — Recent  zinc 
processes  and  furnaces — Cdte-Pierron — Imbert — Johnson — Thierry 
Louvrier-Louis — Electric  smelting  of  other  metals — Copper — 
Nickel —  Tin — Lead. 

CHAPTER  XIII 

MISCELLANEOUS  USES  OF  THE  ELECTRIC  FURNACE 346-365 

Nitric  acid  and  nitrates — Processes  and  furnaces  of  Birkeland  and 
Eyde — Pauling — Schonherr — Calcium  cyanamide — Fused  quartz — 
Glass — Alundum — Phosphorus — Carbon  bisulphide — Monox. 

CHAPTER  XIV 

ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES 366-388 

Electrolysis — Electrochemical  equivalents — Electrolytic  extraction 
processes — Acker  caustic  soda  process — Sodium  processes,  Castner 
— A  shcroft — Carrier —  Virginia  Company — Potassium — Magn  esium 
— Tucker  furnace — Muthmann  crucible — Barium — Strontium — 
Zinc — Swinburne  and  Ashcroft  chlorine  smelting  process — Alumin- 
ium— Electrolytic  refining. 

CHAPTER  XV 

FUTURE  DEVELOPMENTS  OF  THE  ELECTRIC  FURNACE 389-395 

General  considerations — Exhaustion  of  coal  supplies — Use  of  water 
powers — Achievements  of  the  electric  furnace — Probable  future 
uses  of  electric  furnace — Other  sources  of  electrical  power. 

INDEX 397 


INTRODUCTION 

The  rapid  growth  of  the  electric  furnace  makes  it  increasingly 
difficult  for  the  metallurgist  to  keep  in  touch  with  its  recent  devel- 
opments. A  few  years  ago  it  was  a  scientific  curiosity;  now  it 
threatens  to  rival  the  Bessemer  converter,  the  open-hearth  steel 
furnace,  and  even  the  blast  furnace  itself. 

The  halo  of  romance,  that  has  always  surrounded  electricity  in 
all  its  forms,  has  caused  the  wildest  schemes  to  be  originated,  and 
has  given  them  a  hearing;  while,  on  the  other  hand,  practicable 
electric  smelting  processes  have  been  considered  visionary. 

In  this  book,  it  has  been  the  author's  purpose  to  trace  the  evolu- 
tion of  the  electric  furnace  from  its  simplest  beginnings,  and  to  set 
forth,  as  briefly  as  is  consistent  with  clearness,  the  more  important 
facts  relating  to  its  theory  and  practice. 

The  scope  and  arrangement  of  the  book  can  be  gathered  from  the 
titles  of  its  fifteen  chapters.  The  first  is  historical;  four  deal  with 
the  classification,  efficiency,  construction  and  operation  of  electric 
furnaces;  nine  chapters  treat  of  the  various  uses  of  the  electric  fur- 
nace, and  the  last  is  an  attempt  to  look  into  the  future  of  the  electric 
furnace. 


Xlll 


THE  ELECTRIC  FURNACE 

ITS  CONSTRUCTION,  OPERATION  AND  USES 


CHAPTER  I 
HISTORY  OF  THE  ELECTRIC  FURNACE 

The  electric  furnace  is  of  comparatively  recent  origin.  The 
first  of  any  practical  importance,  was  constructed  by  Sir  W.  Siemens 
in  1878,*  and  in  i8822  he  melted  in  an  electric  furnace  some  20  Ib. 
of  steel  and  8  Ib.  of  platinum.  Since  that  time  the  development  has 
been  rapid. 

The  beginning  of  the  electric  furnace  may,  however,  be  traced 
much  farther  back  than  this.  In  1800 — only  a  few  months  after 
Volta's  discovery  of  the  electric  battery — 'Sir  Humphry  Davy, 


FIG.  i. — The  electric  arc. 

experimenting  with  the  new  battery,  produced  the  first  arc  light 
between  carbon  points,3  and,  as  the  electric  arc  is  the  source  of 
heat  in  an  important  class  of  electric  furnaces,  its  discovery  was 
the  first  step  in  their  evolution. 

1  Siemens'  Electric  Furnace,  Journ.  Soc.  of  Telegraph  Engineers,  June,  1880. 

2  Siemens  and  Hunlington,  British  Assoc.  for  the  Adv.  of  Science,  1882,  p.  496. 

3  Davy,  S.  P.  Thompson's  Electricity  and  Magnetism,  Phil.  Trans.  Roy.  Soc., 
vol.  xcvii  (1809),  p.  71,  and  vol.  cxi  (1821),  p.  427. 

1 


FURNACE 

The  electric  arc,  as  shown  in  Fig.  i,  may  be  produced  by  passing 
an  electric  current  through  two  carbon  rods  which  touch  each  other 
and  then  drawing  them  apart.  The  arc  consists  of  a  flame  of  va- 
porized carbon,  extending  from  one  carbon  pole  to  the  other.  When 
an  electric  current  meets  with  resistance,  it  is  transformed  into 
heat,  and,  as  the  carbonaceous  vapor  offers  a  considerable  resistance 
to  the  electric  current,  a  very  high  temperature  is  produced;  high 
enough  to  melt  or  vaporize  any  known  substance. 

In  the  direct- current  arc  the  positive  carbon,  which  is  marked 
+  in  the  figure,  is  hollowed  out  by  the  current,  and  becomes  intensely 
white  hot,  presenting  the  dazzling  bright  light  with  which  all  are 
acquainted.  The  arc  light  is,  in  fact,  a  miniature  electric  furnace 
of  the  arc  type;  and  produces  a  temperature  not  much  inferior 
to  that  in  any  modern  electric  furnace.  It  has  been  supposed  that 
the  hollowing  out  of  the  positive  carbon  is  due  to  an  electrolytic 
conveyance  of  carbon  from  the  positive  to  the  negative  electrode; 
but  recent  experiments  show  that  any  electrical  transfer  of  carbon 
is  in  the  other  direction,  being  a  stream  of  electrons  from  the  nega- 
tive electrode,  like  the  kathode  discharge  in  a  vacuum  tube.  The 
bombardment  of  the  positive  carbon  by  this  stream  of  electrons, 
generates  so  much  heat  that  the  electrode  becomes  white  hot  and 
rapidly  evaporates,  thus  producing  the  characteristic  crater-like 
form. 

This  explanation  appears  to  fit  in  well  with  the  appearance  of 
an  arc  that  has  been  drawn  out  to  a  little  more  than  its  normal 
length.  The  arc  (which  should  only  be  observed  through  a  dark- 
colored  glass  screen)  will  be  noticed  to  stream  freely  from  the  tip 
of  the  negative  electrode,  and  its  starting-point  on  this  electrode 
is  unaffected  by  drafts  or  magnetic  influences.  The  current  passes 
with  difficulty  on  to  the  positive  electrode,  and  does  not  always 
select  the  point  nearest  to  the  negative  electrode,  but  is  blown 
about  and  wanders  over  a  considerable  area  of  the  electrode.  The 
temperature  of  the  hottest  part  of  the  positive  carbon  in  the  electric 
arc  has  been  measured,  and  is  considered  to  be  about  3,600°  C. 
(6,500°  F.),  which  is  twice  the  temperature  of  melting  platinum  or 
melting  quartz,  and  more  than  twice  the  temperature  of  the  open- 
hearth  steel  furnace.1 

In  the  use  of  a  direct- current  arc  for  lighting,  it  is  usual  to  make 

1  For  additional  information  about  electric  arcs  see  W.  S.  Weedon,  "A  Contri- 
bution to  the  Study  of  the  Electric  Arc,"  Trans.  Am.  Electrochem.  Soc.,  v,  1904, 
p.  171. 


HISTORY  OF  THE  ELECTRIC  FURNACE  3 

the  upper  carbon  the  positive  electrode,  in  order  to  throw  the  great- 
est illumination  downward.  In  Fig.  i  this  arrangement  has  been 
reversed,  and  in  this  position  the  positive  carbon  serves  as  a  minia- 
ture cup  in  which  any  substance  can  be  placed  in  order  to  study 
its  behavior  at  these  high  temperatures. 

The  writer  has  placed  a  small  cylinder  of  refractory  material 
around  the  lower  carbon  of  such  an  arc,  and,  with  this  simple 
apparatus,  was  able  to  repeat  some  of  Moissan's  well-known  experi- 
ments on  the  production  of  the  diamond. 

In  another  form  of  electric  furnace,  the  heat  is  produced  by 
the  passage  of  the  electric  current  through  a  solid  or  liquid  conduc- 
tor. This  method  of  producing  electrical  heat  is  typified  in  the 
common  incandescent  lamp.  The  earliest  use  of  this  method 
of  heating  was  in  1815,  when  W.  H.  Pepys1  solved  an  important 
question  in  regard  to  the  nature  of  steel  by  means  of  a  miniature 
resistance  furnace  operated  by  a  battery.  He  placed  some  diamond 
dust  (a  pure  form  of  carbon)  in  a  cut  in  a  piece  of  wrought-iron  wire, 
and  passed  an  electric  current  through  the  wire,  thus  heating  it 
to  redness.  The  iron  -  absorbed  the  diamond  dust  and  became 
converted  into  steel. 

Robert  Hare2  described  in  1839  an  electric  furnace  which  he  had 
constructed  under  the  bell  jar  of  an  air  pump.  The  furnace  was 
operated  in  a  vacuum  by  means  of  an  electric  battery,  and  in  spite 
of  the  very  small  amount  of  power  available,  Hare  succeeded 
in  forming  calcium  carbide  and  graphite,  and  in  isolating  phosphorus 
and  calcium. 

Although  the  principle  of  electric  heating  had  thus  been  discovered 
early  in  the  century,  very  little  progress  was  made  with  the  practical 
application  of  this  source  of  heat  until  the  discovery  of  the  dynamo. 
Among  those  who  attempted  to  utilize  electrical  heat  in  small 
furnaces,  with  the  aid  only  of  powerful  electric  batteries,  may  be 
mentioned — Napier,  who,  in  1845,  produced  a  small  arc  in  a  plum- 
bago crucible,  intending  to  reduce  certain  metals  from  their  ores; 
Despretz,3  who,  in  1849,  made  a  small  tube  of  charcoal,  about  an 
inch  long,  and  heated  it  by  passing  through  it  an  electric  current 

1  Phil.  Trans.  Roy.  Soc.,  1815,  vol.  cv,  p.  371. 

2  Robert  Hare's  Electric  Furnace.     C.  A.  Doremus,  Trans.  Am.  Electrochem. 
Soc.,  xiii,  1908,  p.  347. 

3  Despretz,  Comptes  Rendus  de  1'Acad.  des  Sciences,  vol.  xxviii,  p.  755,  and 
vol.  xxix,  pp.  48,  545,  712  (1849). 

F.  A.  J.  FitzGerald,  Electrochem.  and  Met.  Ind.,  iii,  1905,  p.  215. 


THE  ELECTRIC  FURNACE 


from  a  battery  of  600  Bunsen  cells;  and  Pichou,1  who  described, 
in  1853,  a  furnace,  heated  by  a  series  of  electric  arcs.  The  furnace, 
which  was  probably  never  constructed,  was  intended  for  the  reduc- 
tion of  metallic  ores.  Joule  and  Thomson  also  attempted  to  util- 
ize the  high  temperature  of  the  electric  arc. 

Until  the  invention  of  the  dynamo,  in  1867,  experiments  requiring 
any  considerable  amount  of  electrical  power  could  only  be  conducted 
at  great  trouble  and  expense  by  means  of  electric  batteries.  Sir 
W.  Siemens,  with  the  aid  of  the  dynamo,  began,  in  1878,  to  experi- 


FIG.  2. — Siemens'  vertical  arc  furnace. 

ment  on  the  electric  furnace,  which  he  used  mainly  for  melting  metals. 
The  form  of  furnace  usually  associated  with  his  name2  is  shown 
in  Fig.  2,  and  consists  of  a  crucible  A  of  graphite  or  similar  refrac- 
tory material,  and  of  two  rods,  B  and  C,  for  leading  in  the  current. 
The  lower  rod  was  made  of  metal,  and  fitted  into  the  base  of  the 
crucible,  while  the  upper  was  of  carbon,  or  a  water-cooled  metal 
tube,  and  was  actuated  by  an  automatic  regulating  device  to  main- 
tain the  arc  £  of  a  constant  length.  The  metal  to  be  melted  was 
placed  in  the  crucible,  making  electrical  contact  with  the  lower 

1  Mentioned  by  Andreoli,  Industries,  1893,  see  Borchers'  Electric  Smelting. 

2  W.  Siemens'  English  patent,  2,110,  1879,  see  Borchers'  Electric  Smelting. 


HISTORY  OF  THE  ELECTRIC  FURNACE  5 

pole  C;  then  the  rod  B  was  lowered  until  an  arc  was  started  between 
this  rod  and  the  metal  in  the  crucible.  In  the  illustration  the  metal 
is  shown  melted,  at  D,  as  it  would  be  at  the  end  of  the  operation. 

The  positive  pole  is  always  hotter  than  the  negative  pole,  and 
for  this  reason  the  metal  to  be  melted  is  made  the  positive  pole  of 
the  arc.  A  lid,  F,  was  provided  with  a  hole  for  observing  the  opera- 
tion, or  making  additions  to  the  charge,  and  a  protecting  covering 
G,  was  arranged  to  reduce  as  far  as  possible  the  radiation  of  heat 
from  the  crucible. 


FIG.  3. — Siemens'  horizontal  arc  furnace. 

In  this  furnace  he  was  not  only  able  to  melt  several  pounds  of  steel 
and  platinum,  but  even  to  vaporize  copper  which  had  been  packed 
with  carbon  in  the  crucible.1 

Siemens  also  invented  a  furnace  having  horizontal  electrodes,  as 
shown  in  Fig.  3.2  In  this  furnace  the  arc  passes  between  the  two 
electrodes  B  and  C,  and  heats,  by  radiation,  the  material  contained 
in  the  crucible.  In  both  furnaces  he  provided  water-cooled  copper 
electrodes  for  the  negative  pole  of  the  arc,  to  avoid  the  wasting  that 
takes  place  when  carbon  electrodes  are  used.  In  Fig.  3,  the  negative 
electrode,  C,  consists  of  .a  copper  tube,  closed  at  one  end,  and  cooled 
by  water,  which  is  introduced  by  a  smaller  pipe  inside  it.  The  posi- 
tive electrode,  B,  is  a  hollow  carbon  rod,  and  through  it  a  neutral  or 
reducing  gas,  can  be  introduced  into  the  furnace. 

In  1883,  Faure  patented  an  electric  furnace  of  the  resistance  type, 
the  heat  being  generated  by  the  passage  of  the  current  through 

1  Siemens  and  Huntington,  British  Assoc.  for  the  Adv.  of  Science,  1882,  pp. 
496-98. 

2W.  Siemens'  English  patent,  4,208,  1878,  see  Borchers'  Electric  Smelting. 


6 


THE  ELECTRIC  FURNACE 


solid  conducting  rods  imbedded  in  the  hearth  of  the  furnace,  on  the 
same  principle  as  the  electric  cooking  stove. 

The  resistance  type  of  electric  furnace  was  made  a  commercial 
success  by  the  brothers,  E.  H.  and  A.  H.  Cowles,  whose  inventions 
were  described  in  I885-1  Their  furnace  was  heated  by  passing  an 
electric  current  through  coarsely  powdered  charcoal  or  gas  carbon. 
This  new  method  was  used  for  a  variety  of  purposes,  one  of  these 
being  the  production  of  aluminium  alloys  by  heating  a  mixture  of 
alumina  and  carbon  with  copper  or  some  other  alloying  metal. 

Fig.  4  represents  the  Cowles  furnace  for  aluminium  alloys.  It 
consists  of  a  rectangular  brick  chamber  fitted  with  inclined  carbon 
electrodes,  A  and  B,  and  filled  with  the  mixture  of  alumina,  carbon 


FIG.  4. — Cowles'  furnace  for  aluminium  alloys. 

and  copper.  The  electric  current  flows  between  the  electrodes 
through  some  pieces  of  retort  carbon,  C,  and  thus  heats  the  charge, 
which,  when  heated,  carries  part  of  the  current.  The  gases  resulting 
from  the  chemical  reaction  escape  and  burn  at  D,  and  the  molten 
alloy  collects  at  the  bottom  of  the  furnace. 

In  1886,  Hall,2  and  Heroult3  patented  processes  for  the  production 
of  aluminium,  and  their  processes,  as  now  used,  consist  in  passing 
an  electric  current  through  fused  compounds  of  aluminium;  the  elec- 

*Dr.  T.  Sterry  Hunt,  Amer.  Inst.  Min.  Eng.  (Sept.  16,  1885),  vol.  xiv,  p.  492. 
Prof.  C.  F.  Mabery,  Amer.  Assoc.  for  the  Adv.  of  Science,  Aug.  28,  1885,  vol. 
xxxiv,  p.  136.  E.  H.  and  A.  H.  Cowles.  U.  S.  patents  319,795  (1884),  see 
Borchers'  Electric  Smelting;  and  324,658  and  324,659  (1885),  see  Richards' 
Aluminium. 

2  C.  M.  Hall,  U.  S.  patents  400,766  and  400,664,  April  2,  1889  (applied  for  July 
9,  1886),  see  Richards'  Aluminium. 

3  Paul  Heroult,  French  patents  175,711,  April  23,  1886,  and  170,003,  April  15, 
1887,  see  Richards'  Aluminium. 


HISTORY  OF  THE  ELECTRIC  FURNACE  7 

trolytic  action  of  the  current  liberates  the  aluminium  from  these 
compounds,  and  the  heat  of  the  current  keeps  the  material  fused. 

Fig.  5  may  be  considered  to  represent  either  the  Hall  or  the 
Heroult  furnace.  Each  of  these  consists  of  an  iron  tank,  A,  lined 
with  carbon,  B,  and  provided .  with  a  number  of  carbon  rods,  C, 
which  dip  into  the  fused  electrolyte,  E,  contained  in  the  tank. 
The  carbon  rods  are  made  the  positive  and  the  tank  the  negative 
electrode.  The  electrolyte  consists  chiefly  of  cryolite,  and  alumina — 
the  purified  ore  of  aluminium — is  added  at  intervals.  The  electro- 
lytic action  of  the  current  splits  up  the  alumina  into  aluminium  and 
oxygen;  the  former  collects  in  the  fused  state  at  the  bottom  of  the 


FIG.  5. — Aluminium  furnace. 

tank,  at  F,  while  the  latter  is  liberated  in  contact  with  the  carbon 
rods,  and  consumes  them,  the  loss  of  carbon  being  about  equal  in 
weight  to  the  aluminium  produced. 

It  will  be  noticed  that,  while  the  apparatus  resembles  Siemens' 
vertical- arc  furnace  in  general  appearance,  no  arc  is  formed  in  this 
case.  The  current  flows  through  the  electrolyte  from  the  carbon 
rods  to  the  melted  aluminium,  and  in  doing  so  produces  enough  heat 
to  keep  the  cryolite  in  a  state  of  fusion,  at  a  temperature  of  nearly 
900°  C.  (1,600°  F.). 

All  the  aluminium  at  present  produced  comes  from  the  electric 
furnace.  During  the  year  1905  the  output  of  aluminium  in  the 
United  States  alone  amounted  to  10,000,000  lb.,  whereas,  in  1885 — • 
before  the  electrical  process  was  indented — it  was  only  283  lb.  In 
1911  the  total  production  of  aluminium  was  90,000,000  lb. 


8 


THE  ELECTRIC  FURNACE 


The  next  stage  in  the  history  of  the  electric  furnace  is  marked 
by  the  classical  experiments  and  researches  of  Henri  Moissan.1 
These  researches  were  commenced  in  1892,  and  had  for  their  objective 
the  manufacture  of  artificial  diamonds.  Moissan  worked  in  accord- 
ance with  scientific  method,  and,  although  his  researches  were  not 
conducted  with  a  view  to  technical  results,  his  unique  experiments 
have  given  a  great  impetus  to  the  commercial  use  of  the  electrical 
furnace,  as  well  as  establishing  on  a  scientific  basis  our  knowledge  of 
chemistry  at  the  high  temperatures  used  in  the  electric  furnace. 


FIG.  6. — Moissan's  furnace. 

Fig.  6  indicates  the  type  of  furnace  he  usually  employed.  It 
consists  of  two  blocks  of  limestone,  A  and  B,  and  two  carbon  rods, 
C  and  D,  to  which  the  electrical  connections  are  made.  A  cavity  is 
hollowed  out  in  these  blocks,  and  the  material  to  be  heated  is  placed 
in  a  crucible  of  carbon  or  magnesia.  As  even  lime  melts  and 
volatilizes  at  the  temperature  of  this  furnace,  a  lining  of  alternate 
layers  of  carbon  and  magnesia  was  arranged  as  shown  in  the  figure, 
in  order  to  withstand,  as  far  as  possible,  the  heat  of  the  arc. 

In  some  of  these  experiments  Moissan  converted  two  or  three 
hundred  electrical  horse-power  into  heat  in  a  furnace  of  only  a  few 
inches  internal  dimensions.  At  the  enormously  high  temperature 
of  his  furnace  everything  melts  or  turns  to  vapor.  Carbon  is  the 
most  refractory  substance  known,  and  even  that  turns  to  graphite 

1  H.  Moissan,  Description  d'un  nouveau  four  electrique,  Comptes  Rendus  de 
1'Acad.  des  Sciences,  vol.  cxv,  p.  1031,  Dec.,  1892. 
H.  Moissan,  Le  Four  Electrique,  Paris,  1897. 
H.  Moissan,  The  Electric  Furnace,  trans,  by  Victor  Lenher,  1904. 


HISTORY  OF  THE  ELECTRIC  FURNACE  9 

and  volatilizes;  magnesia,  another  very  refractory  substance,  melts 
at  the  highest  temperature  of  the  furnace  and  vaporizes.  Lime, 
quartz,  and  alumina  all  melt  and  boil  in  the  furnace.  Gold,  copper, 
iron,  and,  in  fact,  all  the  metals  can  also  be  melted  and  boiled  in  the 
electric  furnace. 

An  improved  form  of  the  Moissan  furnace1  has  recently  been 
described,  in  which  an  electric  current  of  1,000  amperes  at  from  50 
to  150  volts  is  employed.  In  the  case  of  direct  current  this  would 
mean  70  to  200  h.p.,  and,  while  this  is  not  quite  as  much  as  Moissan 
sometimes  used,  it  is  more  than  is  often  available  for  scientific  experi- 
mental work.  In  such  a  furnace  it  is  easy  to  produce  a  temperature 
more  than  double  that  usually  obtainable  by  the  combustion  of  fuel, 
and  it  is,  therefore,  an  invaluable  apparatus  in  the  hands  of  the  metal- 
lurgist and  the  chemist. 

Moissan  also  experimented  on  the  reduction  of  metals  from  their 
oxides,  and  found,  as  had,  indeed,  been  stated  by  C.  F.  Mabery2  in 
1885,  and  by  Dr.  W.  Borchers,  in  1891,  that  carbon  will  reduce  any 
metal  from  its  oxide  at  the  temperature  of  the  electric  furnace.  Not 
only  will  carbon  reduce  any  metal  from  its  oxide,  but  at  this  high  tem- 
perature carbon  will  also  combine  with  the  metal  itself  to  form  a 
carbide.  The  production  and  properties  of  many  of  these  carbides 
were  studied  by  Moissan. 

One  of  the  most  spectacular  of  his  experiments  was  the  pro- 
duction of  the  diamond.  This  is  a  crystallized  form  of  carbon, 
and  if  a  suitable  solvent  were  available  it  should  be  possible  to  crys- 
tallize carbon  as  diamonds.  Moissan  found  such  a  solvent  in  iron 
and  certain  other  metals.  In  the  electric  furnace  these  metals  dis- 
solve notable  quantities  of  carbon,  and  by  cooling  them  under  suit- 
able conditions  Moissan  was  able  to  obtain  some  of  the  carbon  as 
microscopical  diamonds,  which  he  isolated  by  dissolving  the  metal 
in  acids.  The  present  writer,  in  common  with  other  experimenters, 
has  repeated  this  production  of  the  diamond,  and  has  also  seen  what 
appeared  to  be  a  diamond,  which  had  been  found  imbedded  in  a 
piece  of  iron  or  steel  produced  by  ordinary  smelting  methods. 

Although  diamonds  are  not  yet  manufactured  in  ton  lots,  Moissan's 
researches  on  the  conversion  of  carbon  into  graphite,  and  on  the  pro- 
duction of  calcium  carbide,  have  been  followed  by  important  com- 
mercial developments.  The  formation  of  calcium  carbide  in  the 
electric  furnace  was  independently  achieved  in  1892  by  T.  L.  Willson, 

1  Engineering,  March  23,  1906,  vol.  Ixxxi,  p.  381. 

2  C.  F.  Mabery,  Amer.  Assoc.  for  the  Adv.  of  Science,  xxxiv,  p.  136. 


10 


THE  ELECTRIC  FURNACE 


who  developed  the  manufacture  of  the  carbide  on  commercial  lines.1 
Fig.  7  illustrates  the  Willson  carbide  furnace,  consisting  of  an  iron 
crucible,  A,  the  base  of  which  has  a  carbon  lining,  D.  The  crucible 
is  connected  to  one  cable  from  the  dynamo  or  transformer,  while 
the  other  cable  is  connected  to  a  large  carbon  electrode,  B  C,  sus- 
pended within  the  crucible.  The  arc  being  started  between  C  and  D, 
the  charge  of  powdered  lime  and  coke  is  fed  in  around  C,  and  in  the 

heat  of  the  arc  the  lime  is  reduced  by 
means  of  the  coke  to  the  metal  calcium, 
and  this  in  turn  reacts  with  more  coke 
to  form  a  carbide.  These  reactions  may 
be  represented  by  the  following  chemical 
equations,  which  also  indicate  the  rela- 
tive amounts  of  lime  and  coke  to  use  in 
the  charge: 

CaO+    C  =  Ca+CO. 


FIG.  7. — Willson's  carbide 
furnace. 


The  calcium  carbide,  thus  formed,  is 
fusible  at  the  temperature  of  this  furnace, 
and  forms  a  pool  beneath  the  electrode, 
B  C,  and  by  gradually  raising  this  elec- 
trode, a  mass  of  carbide  is  built  up. 
When  the  crucible  is  nearly  filled,  the 
operation  is  stopped  and  the  crucible  allowed  to  cool  before  turning 
out  the  block  of  carbide. 

The  carbonic  oxide  produced  by  the  reaction  escapes  and  burns 
in  the  upper  part  of  the  crucible,  as  is  indicated  in  Fig.  7.  Many 
other  forms  of  carbide  furnaces  have  been  devised,  and  are  now  being 
operated  on  a  large  scale,  some  of  these  being  intermittent,  like  the 
Willson  furnace,  while  others  are  continuous  in  action.  The  world's 
production  of  calcium  carbide  amounted  in  1904  to  90,000  tons,  and 
in  1909  to  250,000  tons.  The  value  of  calcium  carbide  depends,  as  is 
well  known,  upon  the  ease  with  which  it  acts  upon  water  to  form  the 
valuable  illuminating  gas,  acetylene;  its  price  in  1910  was  $70 
per  ton. 

Another  important  carbide,  produced  in  the  electric  furnace,  is 
carborundum,  a  carbide  of  silicon,  SiC.  The  discovery  of  carborun- 

1  Industries  and  Iron,  1896,  vol.  xx,  p.  322. 

V.  B.  Lewes,  "Acetylene,"  p.   T6.  states  that  Willson  was  the  first  to  (inten- 
tionally) produce  calcium  carbide  in  the  electric  furnace. 


HISTORY  OF  THE  ELECTRIC  FURNACE 


11 


dum  by  E.  G.  Acheson  in  1891  is  described  by  himself  in  an  inter- 
esting lecture  on  "Discovery  and  Invention."1  Mr.  Acheson  was 
attempting  to  harden  clay  by  impregnating  it  with  carbon  in  an 
improvised  electric  furnace.  After  the  experiment  he  noticed  a 
few  bright  specks  at  the  end  of  the  carbon  electrode.  These  specks 
were  found  to  be  hard  enough  to  cut  not  only  glass,  but  even  the 
diamond  itself,  and  were  the  origin  of  the  important  carborundum 
industry. 

Carborundum  is  made  by  placing  a  mixture  of  sand  and  coke  with 
smaller  amounts  of  saw-dust  and  salt  in  a  fire-brick  chamber,  and 
passing  an  electric  current  through  a  core  of  carbon  placed  in  the 


FIG.  8. — Acheson's  carborundum  furnace. 

middle  of  the  charge.  The  sand,  in  the  charge,  becomes  reduced  to 
silicon,  and  combines  with  carbon  to  form  carborundum,  which,  at 
the  high  temperature  (over  2,000°  C.)  of  the  furnace,  assumes  a 
beautiful,  iridescent,  crystalline  form,  and  is  of  such  extreme  hard- 
ness that  it  has  proved  to  be  a  very  valuable  abrasive.  It  is  now 
widely  used  as  a  grinding  agent  in  the  metal  trades  and  other  indus- 
tries, and  it  is  also  useful  as  a  refractory  lining  for  electric  and  other, 
furnaces,  and  as  a  deoxidizing  addition  in  the  manufacture  of  steel. 
The  furnace  employed2  is  shown  in  Fig.  8,  and  consists  of  two 
permanent  end  walls,  A  and  B,  which  support  large  bundles  of  carbon 
rods,  C  and  D,  in  heavy  bronze  holders.  The  current  is  carried  be- 
tween C  and  D  by  a  core  of  broken  carbon,  E,  and  as  the  charge  does 
not  fuse,  this  core  remains  in  position  until  the  end  of  the  operation. 
LM  is  the  carborundum,  which  surrounds  the  core,  and  HK  is  un- 

1  The  Electric  Journal,  Pittsburg,  1906. 

2  The  Carborundum  Furnace,  F.  A.  J.  FitzGerald,  Electrochemical  Industry, 
vol.  iv,  p.  53,  1906. 


12  THE  ELECTRIC  FURNACE 

converted  charge  which  acts  as  a  heat-retaining  cover.  A  layer  of 
brilliant  graphite  was  usually  found  between  the  core  and  the  crys- 
talline carborundum.  This  graphite  resulted  from  the  decomposi- 
tion of  the  carbide  in  the  hottest  part  of  the  furnace.  From  this 
observation  Acheson  evolved  the  artificial  production  of  graphite, 
which  he  patented  in  I896.1  It  consists  in  heating  coke,  anthracite 
or  other  form  of  carbon  containing  a  small  amount  of  iron  oxide  or 
certain  other  substances.  The  iron  and  other  impurities  in  the  car- 
bon are  volatilized  at  the  high  temperature  of  the  electric  furnace  and 
leave  the  carbon  very  pure  and  converted  into  graphite.  As  much  as 
1,000  electrical  horse-power  is  consumed  in  one  of  these  furnaces, 
producing  a  temperature  of  over  2,200°  C. 

The  manufacture  of  carborundum,  graphite,  siloxicon  and  other 
products  of  the  Acheson  electric  furnaces  at  Niagara  is  more  fully 
described  in  Chapter  XI. 

The  continued  fertility  of  soils  depends  largely  upon  the  nitrogen, 
which  is  removed  in  the  crops,  being  restored  to  them  in  the  form 
of  nitrate  manures  ;  it  being  an  unfortunate  circumstance  that 
plants  are  unable  to  utilize  the  free  nitrogen  of  the  air.  The  exten- 
sive deposits  of  Chile  saltpeter,  of  which  more  than  2,000,000  tons 
are  consumed  annually,  form  the  main  source  of  these  manures, 
as  well  as  of  nitric  acid  and  other  nitrates.  In  view  of  the  small 
number  of  important  deposits  and  the  rapid  increase  in  consump- 
tion, it  is  of  very  great  importance  that  some  other  source  of  nitrates 
should  be  discovered.  Naturally  one'  turns  to  the  unlimited  supply 
of  nitrogen  in  the  atmosphere. 

The  combination  of  oxygen  and  nitrogen  in  the  electric  arc  was 
discovered  by  Priestley  and  Cavendish  more  than  100  years  ago, 
but  some  investigations  by  Crook es  in  1893  appear  to  have  drawn 
attention  to  the  possibility  of  utilizing  this  reaction  for  the  manu- 
facture of  nitric  acid  and  nitrogenous  fertilizers  from  air.  A  number 
of  processes  were  patented  in  the  years  1895-96,  and  the  Bradley 
and  Lovejoy  process  was  tried  at  Niagara  Falls  on  a  commercial 
scale  in  1902.  The  Birkeland  and  Eyde  process,  invented  in  1903, 
was  the  first  to  attain  commercial  success,  and  is  now  in  operation 
in  Norway.  Other  processes  have  since  been  invented,  notably 
those  of  Pauling  and  of  Schonherr,  and  at  the  present  time  nitric 
acid  and  nitrates  for  fertilizers  are  made  electrically  from  the  atmos- 
phere on  a  very  large  scale,  several  hundred  thousand  horse-power 

1  The  Conversion  of  Amorphous  Carbon  to  Graphite,  F.  A.  J.  FitzGerald, 
Journal  of  the  Franklin  Institute,  Nov.,  1902. 


HISTORY  OF  THE  ELECTRIC  FURNACE  13 

being  employed  for  this  purpose.  Another  source  of  nitrogen  for 
the  soil  is  calcium  cyanamide,1  which  is  the  result  of  a  direct  reaction 
between  calcium  carbide  and  nitrogen.  This  process  was  discovered 
about  1895  but  was  not  developed  commercially  until  about  1905. 
It  is  now  being  operated  on  a  large  scale,  more  than  100,000  tons 
a  year  being  produced. 

Calcium  carbide  has  been  one  of  the  most  important  products 
of  the  electric  furnace,  and  its  manufacture  still  consumes  more 
electrical  power  than  that  of  any  other  product.  It  was  a  financial 
crisis  in  the  carbide  industry  that  led  to  the  electric  smelting  of 
iron,  steel,  and  the  other  iron  alloys.2 

Several  years  ago  the  production  of  calcium  carbide  became  larger 
than  the  demand,  and  this  forced  some  manufacturers  to  turn  their 
attention  to  other  methods  of  utilizing  their  electric  furnaces. 
With  this  object  experiments  were  made  in  France  and  elsewhere 
about  the  year  1900  on  the  production  of  ferro- chrome,3  ferro- 
silicon,  and  the  other  ferro-alloys;  and  these  experiments  were  so 
successful  that  not  only  have  the  new  processes  been  able  to  compete 
with  existing  methods,  but,  in  many  cases,  the  electric  product 
has  captured  the  market. 

The  ferros  are  alloys  of  iron,  with  manganese,  chromium,  silicon, 
or  some  other  metal,  and  they  usually  contain  a  notable  amount 
of  carbon,  being,  in  fact,  cast-iron,  in  which  part  of  the  iron  has 
been  replaced  by  another  metal.  Some  of  these  are  used  in  the 
production  of  open-hearth  and  Bessemer  steel,  and  others  for  the 
production  of  special  alloy  steels.  Ferro-nickel,  ferro- tungsten, 
ferro-titanium  and  ferro- molybdenum  have  also  been  employed 
in  steel- making. 

The  carbide  furnaces,  which  were  lined  with  carbon,  were  satis- 
factory for  the  production  of  these  carburized  materials,  but  certain 
changes  were  necessary  before  they  could  be  used  for  the  manu- 
facture of  steel.  In  France,  Heroult,4  and  in  Sweden,  Kjellin5 
succeeded  in  adapting  the  furnace  to  the  production  of  good  quality 
steel  from  scrap  steel,  pig-iron,  etc.;  and  good  crucible  and  special 

1  See  page  308. 

2  Albert  Keller,  The  Application  of  the  Electric  Furnace  in  Metallurgy,  Journ. 
Iron  and  Steel  Inst.,  1903,  No.  i,  p.  161. 

3  Ibid.,  pp.  162  and  166-169. 

4  Heroult  Steel  Furnace.     Electrochemist  and  Metallurgist,  vol.  i  (1901),  p. 
196;  Electrochemical  Industry,  vol.  i  (1902-03),  pp.  63,  287,  449. 

5  Kjellin  Steel  Furnace.     Electrochemist  and  Metallurgist,  vol.  i  (1901),  p.  90; 
Electrochemical  Industry,  vol.  i  (1902-03),  pp.  141,  376,  462,  576. 


14  THE  ELECTRIC  FURNACE 

alloy  steels  have  for  some  years  been  produced  commercially  in 
the  electric  furnace.  The  original  patents  of  these  pioneers  of 
electric  steel- making  were  taken  out  about  the  year  1900,*  just 
100  years  after  the  discovery  of  the  voltaic  battery. 

The  origin  of  the  electric  smelting  of  iron-ores  was,  however, 
somewhat  earlier  than  this.  In  the  year  1898  Captain  Stassano,2 
in  Italy,  patented  his  electrical  furnace  for  smelting  iron-ores, 
and  in  the  following  year  demonstrated  the  working  of  his  process. 
Quite  a  sensation  was  produced  by  his  experiments,  as  although 
it  was  not  surprising  to  learn  that  iron-ores  could  be  smelted  by 
electricity,  the  ordinary  price  of  electric  power  was  so  high  that  it 
appeared  preposterous  to  attempt  to  use  it  in  competition  with 
coke  in  the  blast-furnace. 

It  is  a  matter  of  general  knowledge  that  the  retail  price  of  any 
commodity  is  higher,  and  sometimes  even  several  times  as  high 
as  the  wholesale  price,  or  the  cost  of  production;  but  it  was  probably 
not  generally  realized  until  recently  that  the  small  consumer  of 
electric  light  pays  about  100  times  as  much  for  electricity  as  the 
actual  cost  of  producing  it  from  a  good  water-power.  This  enormous 
difference  had  given  an  exaggerated  idea  of  the  costliness  of  elec- 
trical power,  and  was,  no  doubt,  largely  responsible  for  the  skep- 
ticism with  which  Stassano's  early  experiments  were  received. 
These  experiments  of  Stassano  impressed  on  many  minds  the  finan- 
cial possibility  of  electric  smelting  in  general,  and  a  large  crop  of 
such  processes  followed. 

In  view  of  the  great  importance  to  Canada  of  developing  the  elec- 
tric smelting  of  iron-ores,  the  Canadian  Government  appointed 
in  1903  a  Commission  under  Dr.  Haanel  to  report  on  the  elec- 
trothermic  processes  in  operation  in  Europe  for  smelting  iron-ores 
and  making  steel.  The  Commission  visited  Europe  in  1904  and 
saw  the  Heroult,  Keller  and  Kjellin  furnaces  in  commercial  operation 
making  steel  and  ferro-alloys.  At  Dr.  Haanel's  request  the  produc- 
tion of  pig-iron  from  the  ore  was  also  demonstrated  in  the  Heroult 
and  Keller  furnaces.  A  voluminous  report3  was  published  after 
the  return  of  the  Commission,  and  Dr.  Haanel  was  so  well  satisfied 

1  The  Colby  induction  steel  furnace  was  patented  in  1890.     See  Electrochem- 
ical Industry,  vol.  iii  (1905),  pp.  134,  299,  341,  and  vol.  v  (1907),  p.  232. 

2  Stassano  Steel  Furnace.     Electrochemist  and  Metallurgist,  vol.  i  (1901),  p. 
230;  Electrochemical  Industry,  vol.  i  (1902-03),  pp.  247,  363. 

3  Report  of  the  Commission  appointed  to  investigate  the  different  electro- 
thermic  processes  for  the  smelting  of  iron-ores  and  the  making  of  steel  in  operation 
in  Europe.     Ottawa,  1904. 


HISTORY  OF  THE  ELECTRIC  FURNACE  15 

with  the  possibility  of  smelting  iron-ores  electrically  in  countries 
where  coal  was  scarce  and  water-power  was  abundant  that  he 
obtained  a  further  grant  from  the  Government,  and  with  the  help 
of  Paul  Heroult  carried  out  a  series  of  experiments  during  the 
spring  of  1906  at  Sault  Ste.  Marie  on  the  electric  smelting  of  Cana- 
dian iron- ores.1 

After  Dr.  Haanel's  demonstration  of  electric  ore-smelting,  plants 
for  the  commercial  production  of  pig-iron  in  the  electric  furnace  were 
erected  at  Heroult-on- the- Pitt,  California,  and  at  Welland  in 
Ontario,  Canada.  The  Californian  furnace  was  started  on  the 
4th  of  July,  1907,  but  the  furnace  was  not  satisfactory  and  two  years 
were  spent  in  experimental  work  before  a  successful  furnace  was 
built  by  Prof.  D.  A.  Lyon.  The  production  of  pig-iron  was  less 
likely  to  be  commercially  successful  at  Welland  than  in  California, 
and  the  Welland  plant  was  soon  utilized  for  the  production  of  ferro- 
alloys. 

Sweden  and  Norway  are  countries  particularly  suitable  for  electric 
iron-smelting,  and  experiments  were  started  in  Sweden  by  Messrs. 
Gronwall,  Lindblad  and  Stalhane  shortly  after  the  publication  of 
Dr.  Haanel's  work.  Their  first  satisfactory  furnace,  one  of  700  h.p., 
was  erected  at  Domnarfvet  in  1908,  and  in  1910  a  furnace  of  2,500 
h.p.  was  built  at  Trollhattan  in  Sweden  and  is  in  commercial  opera- 
tion. Still  larger  furnaces  have  been  built  both  in  Sweden  and  in 
Norway  and  the  electric  iron-smelting  industry  is  now  well  established 
in  these  countries. 

The  production  of  steel  from  pig-iron  and  steel  scrap  in  the  elec- 
tric furnace  has  been  in  commercial  operation  since  1902,  but  it  was 
soon  found  that  furnaces  of  the  Heroult  type  could  be  more  use- 
fully employed  for  finishing  steel  that  had  been  made  in  a  Bessemer 
converter  or  open-hearth  furnace  and  which  was  then  transferred 
while  still  molten  to  the  electric  furnace.  A  15- ton  Heroult  furnace 
has  been  in  operation  for  this  purpose  in  south  Chicago  since  1908. 
The  Kjellin  induction  furnace,  although  very  satisfactory  for  melting 
steel,  could  not  be  used  for  refining  it;  but  the  invention  in  1907  of  the 
Rodenhauser  furnace,  which  combines  induction  and  resistance 
heating,  has  overcome  this  difficulty  and  has  greatly  extended  the 
use  of  the  induction  steel  furnace. 

The  direct  production  of  steel  from  iron-ore  in  the  electric  furnace, 

1  Report  on  the  experiments  made  at  Sault  Ste.  Marie,  Ont.,  under  Govern- 
ment auspices,  in  the  smelting  of  Canadian  iron  ores  by  the  electrothermic  proc- 
ess. Ottawa,  1907, 


16  THE  ELECTRIC  FURNACE 

which  was  started  by  Stassano  in  1898,  has  not  made  much  headway; 
but  a  good  deal  of  experimental  work  has  been  done  lately,  among 
others  by  J.  W.  Evans  and  the  author,  and  the  outlook,  in  certain 
directions,  seems  promising. 

Attempts  to  smelt  zinc-ores  electrically  were  made  by  Cowles 
in  1885,  and  the  original  de  Laval  furnace  was  patented  in  1902. 
Since  that  time  many  attempts  have  been  made  with  partial  success 
to  solve  the  problem.  A  modified  de  Laval  furnace  is  in  opera- 
tion in  Sweden  but  the  results  obtained  are  far  from  satisfactory.  W. 
Me  A.  Johnson,  who  has  worked  on  the  problem  for  ten  years,  now 
claims  that  he  has  solved  it.  A  research  was  started  in  the  author's 
laboratory  for  the  Canadian  Government  in  1910  and  is  still  in 
progress,  on  a  larger  scale,  in  the  Government  works  at  Nelson,  B.  C. 

Recently  the  electric  furnace  has  been  employed  in  the  metallurgy 
of  copper  and  of  nickel  and  has  been  found  satisfactory  for  certain 
operations,  but  not  for  smelting  the  arsenical  silver-ores  of  cobalt. 


CHAPTER  II 

DESCRIPTION  AND  CLASSIFICATION  OF  ELECTRIC 
FURNACES 

The  electric  furnace  may  be  described  as  an  appliance  in  which 
materials  can  be  submitted  to  a  high  temperature  by  the  dissipation 
of  electrical  energy.  This  definition  does  not  include  all  cases  of 
electrical  heating;  and  with  advantage  might  be  limited  to  the 
production  of  temperatures  above  a  red  heat.  In  a  number  of 
instances  such  as  the  production  of  sodium  and  aluminium,  the 
electric  current  is  required  mainly  for  isolating  the  metal  by  electroly- 
sis, and  only  incidentally  for  producing  heat.  These  processes  are 
usually  considered  to  be  furnace  operations,  because  a  high  tempera- 
ture is  produced,  and  electrolysis  should  be  classed  as  a  furnace 
process  when  fused  anhydrous  salts  are  employed,  excluding  the 
more  familiar  electrolytic  processes  in  which  aqueous  electrolytes 
are  used. 

Heat  is  produced  whenever  an  electric  current  encounters  any 
resistance  to  its  flow;  the  energy,  producing  the  current,  being 
transformed  into  heat.1  Even  the  best  electrical  conductors 
oppose  some  resistance  to  the  flow  of  an  electric  current,  and  work 
must  consequently  be  done  in  maintaining  the  current.  If  an 
electric  circuit  is  made,  in  part,  of  a  good  conductor  (such  as  a  short, 
stout  copper  cable)  and,  in  part,  of  a  poor  conductor  (such  as  a  thin 
rod  of  carbon)  the  greater  part  of  the  heat  will  be  produced  in  the 
poor  conductor,  which  may  even  become  red  hot,  while  the  remainder 
of  the  circuit  remains  cool. 

Fig.  9  represents  such  a  circuit:  D  is  a  dynamo,  or  electric  gen- 
erator; B  and  C  are  stout  copper  wires  or  cables,  and  R  is  a  carbon 
" Resistance"  or  "Resistor";  that  is  to  say,  an  electrical  conductor 
made  of  carbon  that  offers  a  considerable  resistance  to  the  flow  of 
the  current.  The  windings  in  the  dynamo  are  of  copper,  and  these 
and  the  cables  B  and  C  are  so  stout,  that  the  resistance  they  offer 
to  the  flow  of  the  current  is  only  small.  In  this  circuit,  mechanical 
work  is  constantly  required  to  turn  the  dynamo,  and  this  work  is 
converted  into  heat  mainly  in  the  resistor  R;  and  to  a  less  extent  in 
the  conductors  B  and  C,  and  the  dynamo  D.  Such  an  arrangement 

1  A  part  of  the  energy  is  sometimes  changed  into  chemical  energy  or  into  other 
forms  of  electrical  energy. 

2  17 


18  THE  ELECTRIC  FURNACE 

may  represent  an  electric  resistance  furnace  operated  by  a  dynamo. 
The  work  spent  in  driving  the  dynamo,  is  converted  into  heat,  and 
by  giving  to  the  furnace  a  far  higher  resistance  than  that  of  the  re- 
mainder of  the  circuit,  we  can  obtain  nearly  all  the  heat  in  the  fur- 
nace; only  a  small  proportion  being  wasted  in  the  dynamo  and 
conducting  cables.  The  amount  of  heat  developed  depends  upon 
the  strength  of  the  electric  current,  as  well  as  on  the  amount  of 
resistance  it  meets.  By  increasing  the  furnace  resistance,  the 
current  is  decreased;  consequently,  beyond  a  certain  point,  less  heat 
will  be  produced  in  the  furnace.1 


FIG.  9. — Electric  circuit. 

An  electric  current  is  measured  in  "amperes,"  the  electrical  pres- 
sure producing  the  current  is  measured  in  "volts,"  and  the  electrical 
resistance  of  a  conductor  is  measured  in  "  ohms."  Using  these  units, 
the  electric  current  flowing  around  a  circuit  is  equal  to  the  electrical 
pressure  or  E.M.F.  (electromotive  force)  driving  it,  divided  by 
the  electrical  resistance  of  the  circuit. 

When  an  electric  current  flows  through  a  resistor,  as  in  Fig.  9,  the 
amount  of  heat  produced  is  proportional  to  the  resistance,  and  to  the 
square  of  the  current;  or,  to  the  E.M.F.  and  the  current.  Taking 
as  a  unit  the  heat  that  would  raise  the  temperature  of  i  grm.  of 
water  from  o°  C.  to  i°  C.,  it  is  found  that — 

H  =  o.24l2R  /  =  o.24  E  1 1, 
where — 

H  =  heat  produced  in  gram  centigrade  units, 
7  =  current  in  amperes, 
R  =  resistance  in  ohms 
E  =  electromotive  force  in  volts, 
/  =  time  in  seconds. 

1  For  a  constant  total  voltage,  the  heat  produced  in  the  furnace  will  be  a 
maximum  when  the  furnace  resistance  is  one-half  of  the  total  resistance  in  the 
circuit.  This  would  not  be  a  desirable  condition,  however,  for  then  as  much  heat 
would  be  wasted  in  the  electric  generator  and  the  leads,  as  was  produced  and  used 
in  the  furnace. 


DESCRIPTION  AND  CLASSIFICATION  19 

In  the  circuit  shown  in  Fig.  9  the  current  /  would  be  measured  in 
amperes  by  means  of  an  ammeter,  A ,  placed  in  one  of  the  cables;  the 
E.M.F.,  E,  in  volts  by  means  of  a  voltmeter,  V,  connected  to  the 
terminals  of  the  resistor;  and  the  resistance  R,  in  ohms,  would  be 
deduced  from  the  relation  IR  =  E.  The  above  considerations  are 
only  exact  in  the  case  of  an  electric  current  flowing  steadily  in  one 
direction;  in  the  case  of  alternating  currents  an  electrical  inertia  is 
observed  which  modifies  these  results.1 

In  the  arc-furnace,  the  electric  current  encounters  not  only  an 
inert  resistance,  but  also,  an  opposing  electrical  force.  Both  the 
resistance  and  the  opposing  electrical  force  cause  the  energy  of  the 
current  to  be  turned  into  heat,2  and  to  contribute  to  the  heating  of 
the  furnace.  A  similar  opposing  electrical  force  is  present  in  an 
electrolytic  furnace,  such  as  is  used  for  the  production  of  aluminium. 
In  the  latter  case,  however,  the  work  done  in  overcoming  this  force, 
is  turned  into  chemical  energy  (isolating  aluminium  from  alumina) 
instead  of  into  heat.  In  most  furnace  operations,  chemical  and 
physical  changes  are  produced,  and  these  increase  or  diminish  the 
amount  of  heat  liberated  in  the  furnace. 

An  electric  furnace  consists  of  the  following  essential  parts  and 
accessories: 

(1)  Some  Conducting  Material  Heated  by  the  Passage  of  the 
Current. — This  may  be  a  vapor,  as  in  the  electric  arc;  or  a  solid,  such 
as  coke;  or  a  liquid,  such  as  molten  slag  or  molten  steel. 

(2)  An  Envelope  of  Refractory  Material. — The  walls,  floor  and 
roof  of  a  furnace  are  needed  to  conserve  the  heat,  to  retain  the  charge, 
to  exclude  the  air  and  to  support  the  electrodes  and  the  charging  and 
discharging  apparatus. 

(3)  Electrodes,  or  Conductors  for  bringing  the  Current  into  the 
Furnace. — Carbon  rods  are  usually  employed  for  this  purpose.   They 
are  subjected  to  the  heat  of  the  furnace  at  one  end,  and  at  the  other 
end  must  be  sufficiently  cool  to  permit  of  making  electrical  contact  by 
means  of  special  holders  with  the  cables  bringing  the  current  to  the 
furnace.     In  some  furnaces  electrodes  are  not  needed,  the  current 
being  generated  by  induction  in  the  furnace  itself. 

'(4)  Electrode  Holders. — These  are  usually  metal  clamps  for  hold- 
ing and  making  electrical  contact  with  the  carbon  electrodes;  pro- 
vision being  made  for  preventing  the  excessive  heating  of  the  holder. 

1  See  page  123,  Chapter  V. 

2  Part  of  the  electrical  or  thermal  energy  is  used  in  vaporizing  carbon  from  the 
electrode. 


20  THE  ELECTRIC  FURNACE 

(5)  Charging  and  Discharging  Facilities. — Some  furnaces  are  in- 
termittent in  action,  the  charge  being  added,  heated  in  the  furnace 
and  then  removed,  before  a  fresh  charge  can  be  introduced.     Other 
furnaces  are  continuous  in  action,  involving  the  periodic,  or  con- 
tinuous additions  of  the  raw  material,  and  removal  of  the  products. 

Apart  from  the  furnace  itself,  the  following  operating  factors  have 
to  be  considered: 

(6)  Source  of  Electric  Current. — The  electric  current  is  produced 
by  means  of  a  dynamo  or  electric  generator,  and  as  it  is  usually  sup- 
plied at  a  higher  voltage  than  is  suitable  for  the  furnace,  a  transformer 
may  be  required  to  reduce  the  voltage;  the  amount  of  current  being 
simultaneously  increased  almost  proportionately  to  the  reduction 
in  the  voltage.     The  current  may  be  alternating,  or  direct,  but  an 
alternating  current  is  usually  preferred,  as  it  can  be  transformed  more 
readily  from  one  voltage  to  another.     In  cases  where  electrolysis  is 
required,  as  in  the  production  of  aluminium  or  sodium,  the  direct 
current  can  alone  be  used. 

(7)  Cables,  Measuring  Instruments,  and  Regulating  Devices. — 
Cables  are  used  for  bringing  the  electric  current  from  the  transformer 
or  dynamo  to  the  furnace.     Measuring  instruments,  such  as  amme- 
ters, voltmeters  and  wattmeters  are  used  for  measuring  and  record- 
ing the  current,  electromotive  force  and  electrical  power  supplied  tp 
the  furnace.     Regulating  devices  are  required  for  advancing  the 
electrodes  as  they  are  consumed  in  the  furnace,  and  for  regulating 
by  this  means,  or  in  some  other  way,  the  amount  of  current  flowing 
through  the  furnace. 

CLASSIFICATION 

The  usual  classification  of  electric  furnaces  depends  primarily  upon 
the  nature  of  the  resistor  used  to  develop  the  heat.  Thus  there  are 
arc-furnaces,  in  which  the  heat  is  developed  in  the  electric  arc;  and 
resistance  furnaces,  in  which  the  heat  is  developed  by  the  passage  of 
the  current  through  a  solid  or  liquid  resistor.  The  classification  may 
depend,  also  upon  the  manner  in  which  the  heat  is  transmitted  to  the 
charge;  thus  in  arc-furnaces  the  heating  may  be  direct,  as  in  Siemens' 
vertical-arc  furnace,  in  which  the  metal  to  be  melted  forms  one  pole 
of  the  arc;  or  indirect,  as  in  his  horizontal-arc  furnace,  where  inde- 
pendent electrodes  are  employed,  and  in  which  the  heat  is  transmitted 
from  the  arc  to  the  charge  by  radiation  and  conduction. 

In  resistance  furnaces  the  charge  to  be  heated  may  itself  consti- 


DESCRIPTION  AND  CLASSIFICATION 


21 


tute  the  resistor,  or  else  an  independent  resistor  may  be  employed. 
The  latter  nearly  always  consists  of  a  solid  core,  usually  of  carbon, 
and  it  may  be  surrounded  by  the  charge  that  it  is  to  be  heated,  or  may 
be  imbedded  in  the  walls  of  the  furnace.  A  charge  that  is  to  be 
heated  directly  by  the  passage  of  the  current,  may  be  either  solid  or 
liquid,  and  in  the  case  of  a  liquid  charge,  the  electric  current  may  pro- 
duce heat  merely,  or  may  also  produce  electrolysis. 

The  following  classification  is  based  on  these  considerations,  and 
includes  examples  of  each  class. 

ARC  FURNACES 

The  heat  is  produced  by  one  or  more  electric  arcs. 
(i)  Independent-arc  Furnaces. — -The   arc  is  independent  of  the 
charge  to  be  heated,  being  formed  between  two  or  more  movable 


FIG.  10. — Independent  arc  furnace. 

electrodes.  The  charge  is  heated  by  radiation  from  the  arc,  which 
is  usually  horizontal. 

Fig.  10  shows  such  a  furnace,  consisting  of  a  refractory  chamber, 
AB,  in  which  an  arc,  E,  is  formed  between  the  movable  carbon 
electrodes  C  and  D\  the  material  to  be  heated  being  shown  melted 
atF. 

Moissan's  furnace,  Fig.  6,  Siemens'  horizontal-arc  furnace, 
Fig.  3,  and  Stassano's  steel-making  furnace  are  examples  of  this 
class.  The  Stassano  furnace,  Fig.  109  consists  of  a  chamber  lined 
with  magnesia  bricks,  and  provided  with  three  carbon  electrodes, 
between  which  a  three-phase  arc  plays.  The  ore  or  other  material 


22 


THE  ELECTRIC  FURNACE 


is  placed  in  the  chamber  below  the  level  of  the  arc,  and  is  heated 
by  radiation. 

(2)  Direct-heating-arc  Furnaces. — The  charge  in  the  furnace 
forms  one  pole  of  the  arc  and  is  thus  heated  directly  as  well  as  by 
radiation.  The  arc  is  usually  vertical. 

Fig.  ii  represents  an  arc  furnace  in  which  the  material  D,  to  be 
heated,  forms  one  pole  of  the  arc.  A  is  a  chamber  lined  with 


FIG.  ii. — Direct-heating  arc  furnace. 

refractory  material,  anc1  B  and  C  are  the  two  electrodes:  the  upper 
one,  B,  is  movable;  the  lower,  C,  is  fixed,  forming  part  of  the  bottom 
of  the  furnace,  and  making  electrical  contact  with  the  charge  D. 
The  furnace  is  started  by  lowering  B  until  it  touches  D,  thus  allow- 
ing the  current  to  pass.  B  is  then  raised,  forming  an  electric  arc 
between  B  and  D. 

Siemens'  vertical- arc  furnace,  Fig.  2,  Willson's  carbide  furnace, 
Fig.  7,  and  the  Keller  furnace,  Fig.  99,  are  examples  of  this  class. 

In  each  of  these  there  is  one  movable  electrode  and  one  arc,  and  the 
hearth  of  the  furnace  is  made  conducting,  serving  to  lead  the  electric 


DESCRIPTION  AND  CLASSIFICATION 


23 


current  to  the  molten  charge  in  the  furnace.  The  large  Girod  furnace, 
Figs.  97  and  98,  has  four  movable  electrodes  and  an  equal  number 
of  arcs,  but  these  are  all  connected  in  parallel,  and  are  equivalent 
to  one  large  electrode  and  arc;  the  voltage  being  that  of  a  single- 
arc  furnace. 

The  "Electro-Metals"  steel  furnace,  Fig.  101,  is  in  the  same  class, 
although  the  two  arcs  are  not  connected  in  parallel,  but  are  operated 
by  the  separate  phases  of  a  two-phase  current.  In  this  case  the 
voltage  of  the  furnace  is  that  of  a  single- arc  furnace,  and  the  bottom 
serves  as  one  electrode. 


FIG.  12. — Direct-heating  series-arc  furnace. 

Fig.  12  shows  a  direct-heating-arc  furnace  having  two  arcs  in 
series.  A  is  a  chamber  lined  with  refractory  material,  and  B  and 
C  are  two  electrodes,  which  are  both  movable,  and  are  connected 
to  opposite  poles  of  the  electrical  supply.  The  furnace  bottom  is 
not  used  as  an  electrode.  As  the  arcs  are  in  series,  the  furnace 
voltage  includes  that  of  both  the  arcs,  and  will  generally  be  greater 
than  in  a  single-arc  furnace.  It  should  be  noted  that  a  conducting- 
hearth  furnace,  in  which  the  hearth  forms  one  electrode,  cannot 
have  two  arcs  in  series. 

Examples  of  this  class  are  the  Heroult  steel  furnace,  Fig.  93, 
the  Willson  carbide  furnace,  Fig.  121,  and   the  alundum  furnace  * 
Fig.  143-- 

The  Heroult  steel  furnace,  Fig.  93,  consists  of  a  chamber  for  con- 
taining the  molten  steel,  with  two  vertical  carbon  rods  dipping 
through  holes  in  the  roof.  An  arc  is  formed  between  each  carbon 
rod  and  the  fused  charge;  the  current  entering  through  one  rod, 


24  THE  ELECTRIC  FURNACE 

passing  through  the  melted  steel  and  slag,  and  returning  through 
the  other  rod.  The  three-phase  Heroult  furnace,  Fig.  94,  is  also 
an  example,  as  the  hearth  is  not  an  electrode,  and  as  the  electric 
current  passes  through  two  arcs  in  series. 

It  will  be  clear,  from  what  has  been  said,  that  the  direct-heating- 
arc  furnaces  form  two  distinct  classes  which  are  fundamentally 
different  from  a  structural  and  electrical  point  of  view.  These 
may  be  termed  (^4)  the  single- arc  or  electrode-hearth  class  (Fig. 
n),  and  (B)  the  double-arc  class  (Fig.  12).  It  would  be  impossible 
to  place  three  or  four  arcs  in  series  in  a  direct-heating-arc  furnace 
unless  it  were  provided  with  two  hearths,  electrically  separate 
from  each  other. 

Furnaces  of  this  class  are  rather  less  convenient  for  scientific 
investigations  than  the  independent- arc  furnace;  because  the 
temperature  is  less  easy  to  regulate,  the  arc  is  more  difficult  to  con- 
trol (when  the  charge  consists  of  cold  metal),  and  the  carbon  of 
the  electrodes  is  apt  to  affect  the  chemical  composition  of  the  charge. 
On  the  other  hand,  the  heat  is  transmitted  more  directly,  thus 
obtaining  a  greater  economy,  and  only  one  movable  electrode  is 
needed  for  each  arc. 


RESISTANCE  FURNACES 

In  these,  the  heat  is  produced  by  the  passage  of  the  electrical 
current  through  some  solid  or  liquid  resistor.  They  may  be  divided 
into  two  main  classes,  in  one  of  which  a  special  resistor  is  provided, 
and  in  the  other  the  charge  itself  constitutes  the  resistor.  The 
second  class  may  be  subdivided  into  two;  in  one  of  these  the  current 
is  used  merely  to  heat  the  charge,  while  in  the  other  it  also  produces 
electrolysis  of  the  fused  contents  of  the  furnace.  These  will  be 
treated,  for  convenience,  as  three  independent  classes. 

I.    Furnaces  with  Special  Resistor 

The  resistor  is  a  solid,  and  is  imbedded  in  the  walls  of  the  furnace, 
or  in  the  charge  itself. 

(i)  Furnaces  with  the  Resistor  Imbedded  in  the  Walls.— The 
furnace  shown,  in  Fig.  13  may  be  taken  as  an  example;  it  consists 
of  a  tube  T,  often  of  porcelain,  a  spiral  of  platinum  wire,  and  a 
heat- retaining  envelope  or  covering.  An  electric  current  passes 
through  the  wire  and  heats  it  to  any  desired  temperature  below  its 
melting-point,  1,755°  C.,  or  3>2°°°  F.,  and  ultimately  the  tube  and 


DESCRIPTION  AND  CLASSIFICATION 


25 


its  contents  may  be  heated  nearly  to  the  same  temperature.  The 
substance  to  be  heated  is  placed  in  the  tube  T.  This  arrangement 
is  convenient  for  heating  a  material  in  any  particular  gas,  and  for 
observing  the  operation;  as  this  can  be  done  through  glass  or  mica 
windows  at  the  ends  of  the  tube.  Provision  must  be  made  for  pre- 
venting the  displacement  and  short-circuiting  of  the  coils  of  wire 
when  expanded  by  the  heat.  The  temperature  that  can  be  attained 
in  this  furnace  depends  upon  the  refractory  qualities  of  the  tube 
and  envelope,  as  well  as  on  the  melting-point  of  the  platinum  itself, 
and  in  practice  the  temperature  attained  would  be  far  short  of  the 
melting-point  of  the  platinum  wire.1 


FIG.  13. — Electrical  tube  furnace. 

This  furnace  is  very  convenient  for  laboratory  experiments  on  a 
small  scale,  and  at  moderate  temperatures,  but  its  use  is  restricted 
by  the  high  price  of  platinum.2  A  somewhat  similar  furnace  in 
which  the  use  of  platinum  has  been  avoided  is  shown  in  Fig.  14, 
which  represents  in  sectional  elevation,  and  in  plan  with  the  cover, 
B,  removed — a  small  electrical  crucible  furnace,  constructed  at 
McGill  University,  and  intended  for  melting  small  quantities  of  metals. 
It  could,  however,  be  made  considerably  larger,  and  be  used  for 
brass  or  steel  melting.  The  furnace  consists  of  two  fire-clay  blocks 
A  and  B,  a  crucible  C,  and  carbon  electrodes  D  and  E.  A  receptacle 
is  formed  in  the  block  A  to  contain  the  crucible  and  electrodes,  and 
broken  coke,  F,  is  packed  around  them.  The  current  passes  from 
D  to  E  through  the  coke,  which  becomes  hot  and  heats  the  crucible 

1  A  furnace,  in  which  a  crucible  of  fused  quartz  is  surrounded  by  heating-coils 
of  platinum  strip,  has  been  patented  by  W.  H.  Bristol,  Electrochem.,  Ind.,  vol. 
v,  P-  55- 

2  These  furnaces  can  be  obtained  in  several  forms  from  dealers  in  chemical 
apparatus.     A  furnace  suitable  for  heating  a  small  crucible  (Fig.  65)  is  described 
by  Prof.  H.  M.  Howe  in  his  "Metallurgical  Laboratory  Notes,"  p.  37. 

Similar  furnaces  are  now  made  with  heating  coils  of  nichrome  or  other  cheap 
metal  or  alloy. 


26 


THE  ELECTRIC  FURNACE 


and  its  contents.  The  temperature  can  be  regulated  by  a  rheostat 
in  series  with  the  furnace.  The  whole  furnace  is  enclosed  in  a  metal 
box  with  a  thick  asbestos  lining  to  lessen  the  loss  of  heat.1 

Furnaces  of  this  type  can  now  be  made  more  satisfactorily  by 
the  use  of  a  special  resisting  material  called  kryptol2  which  would 
replace  the  coke  in  the  above  description. 

The  Girod  crucible  furnace3  is  constructed  on  the  same  principle, 
and  the  Conley4  ore-smelting  furnace  is  a  large-scale  example  of 
this  class.  One  form  of  the  Conley  furnace  consists  of  a  shaft  down 


FIG.  14. — Electric  crucible  furnace. 

which  the  ore  passes  and  of  carbon  resistors  imbedded  in  the  walls 
of  the  furnace.  The  resistors  are  heated  by  the  passage  of  a  current, 
and  communicate  their  heat  to  the  ore  passing  over  them. 

Small  tube  furnaces  heated  by  spirals  of  platinum  wire,  are  very 
useful  for  experimental  purposes,  but  commercial  furnaces  on  these 
lines  have  been  less  successful.  This  is  mainly  on  account  of  the 
difficulty  of  maintaining  the  resistors  and  adjacent  parts  of  the 
furnace,  and  because,  of  the  slow  conduction  of  heat  to  the  charge, 
ahd  the  large  loss  of  heat  through  the  furnace  walls. 

A  rotary  electric  furnace,  the  inner  walls  of  which  serve  as  resistors, 
being  sufficiently  conducting  when  heated,  has  been  patented  by 
B.  von  Ischewsky.5 

1  Similar  furnaces  have  been  described  by  FitzGerald,  Electrochemical  In- 
dustry, vol.  iii,  pp.  55  and  135. 

2  Kryptol,  see  p.  291. 

3  Girod  furnace,  Electrochem.  Ind.,  ii,  1904,  p.  309. 

4  Conley  furnaces,  Electrochemical  Industry,  vol.  i,  p.  426  and  vol.  ii,  p.  424. 

5  Ischewsky  furnace,  Electrochemical  Industry,  vol.  v,  p.  141. 


DESCRIPTION  AND  CLASSIFICATION 


27 


Tube  furnaces  for  experimental  work,  in  which  the  tube  is  com- 
posed of  graphite,  amorphous  carbon,  or  other  conducting  material 
which  is  heated  by  the  passage  of  the  electric  current,  have  been 
employed  by  Potter,1  Harker  (Fig.  70), 2  Hutton,3  Tucker,4  and 
others.  Some  of  these  are  described  in  Chapter  VI. 

Fig.  15  shows  diagramatically  the  construction  of  a  furnace 
devised  by  Thomson  and  FitzGerald,6  which  has  a  carbon  resistor 
in  the  roof.  This  resistor  consists  of  a  number  of  specially  shaped 
blocks  of  carbon  which  form  an  arch  between  the  electrodes  B  and  C. 


FIG.  15.  —  Thomson-  FitzGerald  resistance  furnace. 


This  arch  may  form  the  roof  of  the  furnace,  or  a  false  roof  F  of 
refractory  tiles  may  be  used  to  protect  the  resistor  from  any  oxidizing 
gases  that  may  be  in  the  furnace.  A  heat-retaining  layer  of  magne- 
sia or  similar  material  is  placed  above  the  arch  and  beneath  the  outer 
cover  D. 

G  is  the  charge,  which  has  been  melted  in  the  furnace,  by  heat 
which  has  radiated  from  E  and  has  passed  through  the  roof  of  tiles, 
F.  Another  form  of  this  furnace  is  shown  in  Fig.  134. 

(2)  Furnaces  with  the  Resistor  Imbedded  in  the  Charge.  —  The 
resistor  is  usually  of  carbon  and  horizontal. 

The  simplest  example  is  Borchers'  experimental  resistance  furnace, 

1  H.  N.  Potter,  Electrochemical  Industry,  vol.  i,  pp.  187,  188  and  250;  vol.  ii, 
p.  203;  vol.  iii,  p.  346,  and  vol.  iv,  p.  191. 

2  J.  A.  Harker,  Electrochemical  Industry,  vol.  iii,  p.  273. 

3  R.  S.  Hutton  and  W.  H.  Patterson,  Electrochemical  Industry,  vol.  iii,  p.  455 


4  S.  A.  Tucker,  Electrochemical  Industry,  vol.  v,  p.  227  (1907). 

5  Thomson  and  FitzGerald,  Electrochem.  and  Metall.  Ind.,  vol.  viii,  1910, 
pp.  289  and  317. 

6  F.  A.  J.  FitzGerald,  "A  New  Electric  Resistance  Furnace,"  Trans..  Am.  Elec- 
trochem. Soc.,  xix,  1911,  p.  273. 


28 


THE  ELECTRIC  FURNACE 


Fig.  i6/  in  which  a  thin  pencil  of  carbon  C  is  supported  between 
stout  carbon  rods  A  and  B,  and  the  charge  to  be  heated  surrounds  C. 
The  current  flows  between  A  and  B  through  C,  and  may  raise  the 
latter  to  a  white  heat.  The  charge  serves  in  part  as  an  envelope 
to  retain  the  heat. 

Acheson's  carborundum  furnace,  Fig.  8,  is  the  most  important 
example  of  this  class.  In  this  furnace  the  conducting  core  is  com- 
posed of  granular  carbon,  and  is  supported  and  surrounded  by  the 
material  to  be  heated.  The  furnace  is  efficient,  because  the  heat  is 
developed  in  the  midst  of  the  charge,  which  serves  to  retain  it.  The 


FIG.  1 6. — Borchers'  resistance  furnace. 

temperature  can  also  be  exactly  regulated  by  varying  the  current, 
while  by  using  a  number  of  cores,  as  in  the  siloxicon  furnace,  Fig.  120, 
it  is  possible  to  obtain  a  fairly  uniform  temperature  throughout  a 
large  portion  of  the  charge.  On  the  other  hand,  when  the  furnace 
is  in  operation,  it  is  impossible  to  regulate  the  resistance  of  the  core,2 
and  since  this  decreases  considerably  as  the  furnace  becomes  hotter, 
the  current,  if  supplied  at  a  constant  voltage,  may  increase  during 
the  work  of  the  furnace  until  it  becomes  too  great  for  the  dynamo, 
or  transformer  from  which  it  is  supplied;  thus  involving  the  use  of 
special  apparatus  for  regulating  the  voltage.  As  the  material  to  be 
heated  acts  as  an  envelope  to  retain  the  heat,  and  as  the  charge 
does  not  become  fused,  the  outer  walls  can  be  of  the  simplest  descrip- 
tion; merely  serving  to  retain  the  charge  in  position.  This  furnace 
could  not  be  used  if  the  charge  were  to  fuse,  since  the  core  would 

1  Borchers'  Electric  Smelting  and  Refining,  1897  Ed.,  Figs.  54,  55,  172,  and 
Electrochemical  Industry,  vol.  iii,  p.  215. 

2  In  small  furnaces  of  this  type  the  resistance  of  the  core  can  be  regulated, 
within  moderate  limits,  by  placing  weights  on  the  charge. 


DESCRIPTION  AND  CLASSIFICATION 


29 


break  and  the  operation  would  stop.  The  furnace  is  also  essentially 
intermittent  in  action,  as  the  charge  cannot  pass  continuously 
through  it,  and  on  that  account  it  is  less  efficient,  since  it  must  be 
allowed  to  cool  between  successive  operations.  Although  a  core  is 
provided  in  this  furnace  to  carry  the  current,  a  portion  of  the  latter 
is  undoubtedly  carried  by  the  charge  itself. 


FIG.  17. — Tone's  resistance  furnace. 

In  the  Cowles  furnace  for  aluminium  alloys,  Fig.  4,  the  charge 
becomes  partly  fused,  and  no  doubt  serves  to  carry  the  current, 
but  at  the  beginning  of  the  operation  the  current  is  carried  by  a 
carbon  core  and  so  the  furnace  may  be  included  in  this  class. 

Tone's  resistance  furnace,1  for  the  reduction  of  metals  is  shown 
in  Fig.  17.  The  central  resisting  core  C  is  placed  vertically  in  order 
to  permit  of  continuous  charging,  which  would  break  down  a  hori- 
zontal core.  It  is  constructed  of  carbon  blocks,  piled  upon  each 

1  F.  J.  Tone,  U.  S.  patent  754,122,  see  Electrochemical  Industry,  vol.  ii  (1904), 
p.  in. 


30 


THE  ELECTRIC  FURNACE 


other  so  as  to  form  a  hollow  square  tower  with  openings  in  the  sides, 
thus  obtaining  a  high  electrical  resistance,  and  offering  a  large 
heating  surface  to  the  charge.  A  and  B  are  carbon  electrodes 
for  making  electrical  connection  with  the  core.  The  charge  is  fed 
in  around  C,  and  the  reduced  and  melted  metal  flows  through  holes 
at  the  base  of  the  furnace  into  the  receptacles,  D  and  E. 

n.  Furnaces  without  Special  Resistor  and  without  Electrolytic 

Action 

In  these  furnaces  the  material  to  be  heated  forms  the  resistor, 
and  may  be  solid  or  liquid,  or  may  become  molten  during  the 
operation.  They  may  accordingly  be  divided  into  three  classes: — 


FIG.  18. — Shaft  furnace  with  lateral  electrodes. 

(i)  Furnaces  with  Solid  Resisting  Contents. — The  material  to 
be  heated  in  these  furnaces  is  sufficiently  conducting  to  serve  as  a 
resistor,  and  remains  solid  during  the  operation  of  the  furnace. 
Such  furnaces  are  in  consequence  usually  intermittent  in  action, 
the  charge  being  heated  and  allowed  to  cool  before  it  can  be  removed 
from  the  furnace. 

The  Acheson  graphite  furnaces,  for  the  manufacture  of  graphite 
from  anthracite  coal,  Fig.  115,  andforgraphitizing  carbon  electrodes, 
Fig.  116  may  be  mentioned  as  members  of  this  class,  although  in 
the  first  a  core  is  needed  to  carry  the  current  when  the  furnace  is 


DESCRIPTION  AND  CLASSIFICATION 


31 


cold,  and  in  the  second  the  heat  is  mostly  produced  in  the  broken 
coke  between  the  piles  of  electrodes.  In  each  case  the  resulting 
graphite,  being  quite  infusible,  remains  in  position  in  the  furnace, 
which  must  therefore  be  allow.ed  to  cool  before  the  charge  can  be 
removed.  Other  examples  are  the  Cowles  zinc  furnace,  Fig.  126, 
the  Johnson  zinc  furnace,  Fig.  127  and  the  Thomson  electric  welding 
apparatus. 

(2)  Furnaces  with  Melting  Resisting  Contents. — The  great 
majority  of  electric  smelting  furnaces  are  in  this  class.  The  current 
passes  through  the  contents  of  the  furnace,  and  these  contents 


FIG.  19. — Shaft  furnace  with  central  electrodes. 

melt  and  run  down  in  the  furnace.  Such  furnaces  are  almost  invari- 
ably continuous  in  action,  fresh  material  being  supplied  at  inter- 
vals, and  the  molten  products  being  tapped  off  while  the  furnace  is 
running.  Almost  all  materials,  when  in  a  melting  condition,  are 
sufficiently  conducting  to  carry  the  current,  although  they  may 
scarcely  conduct  at  all  when  cold. 

In  these  furnaces  the  current  may  pass  between  two  or  more 
lateral  electrodes  as  in  Fig.  18,  or  it  may  pass  from  one  or  more 
movable  electrodes  to  a  fixed  electrode  forming  part  of  the  bottom 
of  the  furnace  as  in  Fig.  19.  The  furnace  illustrated  in  Fig.  18, 


32  THE  ELECTRIC  FURNACE 

consists  of  a  chamber  provided  with  lateral  carbon  electrodes  and 
one  or  more  tapping  holes.  It  has  a  striking  resemblance  to  a  blast- 
furnace, the  electrodes  representing  the  tuyeres.  The  ore  becomes 
heated  and  reduced  to  the  metallic  state  in  the  upper  part  of  the 
furnace,  and  the  whole  charge  melts  in  the  zone  between  the  elec- 
trodes, and  can  be  tapped  out  at  the  bottom.  The  current  passes 
in  part  through  the  molten  slag  and  metal  in  the  bottom  of  the  fur- 
nace, as  well  as  directly  through  the  melting  ore  between  the  two 
electrodes.  In  this  type  of  furnace  the  current  cannot  be  effectively 
regulated  by  moving  the  electrodes,  and  the  walls  are  apt  to  melt 
where  they  touch  the  electrodes  and  the  charge. 

The  Harmet  furnace,  Fig.  80,  and  the  Swedish  iron-furnaces, 
Figs.  90  and  92,  are  examples  of  this  class.  In  the  Swedish  furnaces, 
the  ore  does  not  touch  the  walls  at  the  points  where  the  electrodes 
enter,  and  so  corrosion  is  prevented. 

Fig.  19  represents  a  furnace  with  one  large  electrode,  hung  in 
the  middle,  surrounded  by  the  material  to  be  heated.  The  other 
electrode,  B,  is  fixed,  forming  part  of  the  bottom  of  the  furnace; 
and  merely  serves  to  make  electrical  contact  with  the  fused  material 
in  the  furnace.  An  advantage  in  this  furnace  is  that  the  current 
can  be  easily  regulated  by  raising  or  lowering  the  upper  electrode. 
Moreover,  the  hottest  part  of  the  charge  is  in  the  middle  of  the 
furnace,  thus  leading  to  a  greater  economy  of  heat  and  to  a  longer 
life  of  the  furnace  walls.  On  the  other  hand,  the  upper  electrode 
may  need  to  be  very  long  and  will  be  corroded  by  contact  with  the 
ore  and  furnace  gases. 

The  Heroult  ore-smelting  furnace,  Fig.  78,  the  Haanel-Heroult 
furnace,  Fig.  81  and  the  Salgues  zinc  furnace,  Fig.  129,  are  in  this 
class. 

Comparing  these  with  the  arc- furnaces,  it  will  be  noticed  that  the 
furnace  in  Fig.  18  is  similar  to  an  independent- arc  furnace,  Fig.  10, 
and  that  the  furnace  in  Fig.  19  resembles  the  electrode-hearth  arc- 
furnace,  Fig.  ii.  Another  class  of  resistance  furnaces  could  be  made, 
resembling  the  double- arc  furnace,  Fig.  12,  but  it  would  be  difficult 
to  distinguish  clearly  between  this  and  the  furnace  of  Fig.  18  as 
modified  in  the  Swedish  iron-furnace.  In  resistance  furnaces  with 
solid  or  partly  melted  resisting  contents,  arcs  may  frequently  form 
between  the  electrodes  and  the  charge,  or  in  the  charge  itself,  and  it 
is  therefore  difficult  to  distinguish  certainly  between  a  resistance 
and  an  arc- furnace.  An  electric  smelting  furnace  may  heat  by 
resistance,  when  smelting  an  easily  fusible  charge,  such  as  iron  ore, 


DESCRIPTION  AND  CLASSIFICATION 


33 


which  has  a  low  resistivity,  or  by  an  arc,  when  a  less  fusible  charge 
having  a  high  resistivity  is  used,  as  in  making  calcium  carbide. 

(3)  Furnaces  with  Liquid  Resisting  Contents. — -These  consist  of 
a  refractory  reservoir,  containing  fused  slag,  or  metal,  through 
which  the  electric  current  passes.  The  liquid  becomes  super- 
heated by  the  passage  of  the  current,  and  is  able  to  melt  the  fresh 
material,  which  can  be  added  at  intervals  or  continuously.  The 
current  is  introduced  by  carbon  electrodes,  by  water-cooled  metal 
electrodes,  or  by  induction. 


FIG.  20. — De  Laval  ore-smelting  furnace. 

The  de  Laval  furnace,  Fig.  20,  is  in  this  class;  it  consists  of  a 
chamber,  A,  the  lower  part  of  which  is  divided  into  two  troughs, 
B  and  C,  containing  molten  metal,  with  which  electrical  contact  is 
made  by  metal  terminals.  A  molten  slag,  E,  fills  the  furnace  above 
the  dividing  wall,  and  the  electric  current  flows  between  B  and  C 
through  the  molten  slag.  The  slag  becomes  superheated  and  dis- 
solves the  ore,  F,  which  is  added  through  a  hole,  K,  in  the  top  of  the 
furnace.  Alternating  current  should  be  employed  to  avoid  electroly- 
sis. The  slag  fills  the  furnace  up  to  the  hole,  D,  at  which  it  overflows. 
The  metal  in  the  troughs  overflows  at  the  spouts,  G  and  H,  as  fast 
as  it  is  formed.  In  order  to  prevent  the  melting  away  of  the 
wall  between  the  troughs  a  water-cooled  metal  block,  /,  is  inserted. 
Even  with  this  precaution  there  is  danger  of  short-circuiting,  because 
the  metal  in  B  and  C  may  penetrate  to  the  water  jacket,  /,  thus 
forming  a  complete  metallic  connection  between  the  furnace 
terminals. 

3 


34  THE  ELECTRIC  FURNACE 

The  Snyder  induction  smelting  furnace,  Fig.  131,  resembles  the 
Laval  furnace,  but  the  electric  current  is  generated  within  the 
furnace,  by  induction,  instead  of  being  led  in  by  metal  terminals  or 
electrodes. 

Furnaces  having  resistors  of  liquid  metal  are  used  in  electric  steel- 
making.  Such  a  furnace  consists  of  a  long  canal  containing  the  mol- 
ten steel  which  becomes  heated  by  the  passage  of  the  electric  current. 
The  canal  may  be  folded  backward  and  forward  for  compactness, 
and  to  reduce  the  loss  of  heat.  The  current  may  be  led  in  through 
water-cooled  metal  terminals,  as  in  Gin's  furnace,  Fig.  108;  but  it 
is  preferably  generated  directly  in  the  molten  metal  by  induction, 
the  canal  forming  the  short-circuited  secondary  winding  of  a  trans- 
former, as  in  the  Kjellin  steel  furnace,  Fig.  102;  the  Gronwall  induc- 
tion furnace,  Fig.  104,  and  the  Colby  steel  furnace,  Fig.  103,  and 
Frontispiece. 

Dr.  Bering's  furnace,  Fig.  2I,1  consists  of  a  crucible  or  hearth  con- 
taining molten  metal,  and  two  relatively  small  channels  or  holes  con- 
taining the  same  metal.  These  channels  lead  to  metal  electrodes, 
which  have  water-cooled  terminals  connecting  directly  with  the  low- 
tension  winding  of  a  transformer.  Practically  all  the  heat  in  the 
furnace  is  produced  by  the  passage  of  the  electric  current  through 
the  molten  contents  of  these  constricted  channels  or  holes,  partly  on 
account  of  the  electrical  resistance  of  the  metal,  and  partly  by 
frictional  heating  due  to  the  very  active  circulation.  The  peculiar 
feature  of  the  furnace  is  that  the  heat,  which  is  produced  in  these 
heaters,  is  conveyed  to  the  charge  in  the  crucible  by  an  electrically 
produced  circulation.  This  circulation,  which  is  shown  by  arrows 
in  the  figure,  consists  of  an  outward  flow  of  metal  in  the  center  of 
each  channel,  cooler  metal  returning  down  the  sides  of  the  channel. 
This  circulation  is  not  due  to  differences  of  density  causing  the 
hotter  material  to  rise  in  the  channel,  which  would  be  entirely  too 
slow  for  a  practicable  furnace;  it  is  caused  by  a  newly  discovered 
force  produced  by  the  electric  current  itself.  The  liquid  is  caused 
by  this  internal  electro-mechanical  force,  to  move  radially  from  the 
walls  of  the  channel  to  its  axis,  and  by  the  hydraulic  pressure  thus 
produced,  it  then  escapes  along  the  axis  and  through  the  open  end 
into  the  body  of  the  furnace,  producing  what  has  been  popularly 
termed  the  "squirt  effect."  The  same  force  produces  the  well- 

xDr.  Carl  Hering,  Trans.  Am.  Electrochem.  Soc.,  vol.  xix,  1911,  p.  255.  The 
author  is  indebted  to  Dr.  Hering  for  the  illustration  and  for  some  of  the  informa- 
tion contained  in  this  account. 


DESCRIPTION  AND  CLASSIFICATION 


35 


WorferCoqled 
Terminal 


FIG.  21. — Bering's  resistance  furnace. 


36  THE  ELECTRIC  FURNACE 

known  "pinch  effect"  (see  page  137),  in  the  induction  furnace  and 
in  liquid  conductors  in  general.  The  rapidity  of  the  circulation  is 
indicated  in  the  figure  by  the  mound  of  molten  metal  abova  each 
channel,  and  is  under  the  control  of  the  designer. 

A  furnace  of  this  construction  requires  a  large  current  at  a  low 
voltage,  and  this  is  best  supplied  by  a  special  transformer  having 
a  secondary  winding  of  only  one  turn,  connected  directly  to  the  elec- 
trode terminals;  in  tilting  furnaces  the  transformer  is  attached  to 
the  outside  of  the  furnace. 

In  Gin's  furnace,  or  Kjellin's  furnace,  to  obtain  a  sufficiently 
high  electrical  resistance,  the  whole  of  the  metal  to  be  heated  is  con- 
tained in  a  long  channel  of  small  section  and  this  is  an  unsatisfactory 
feature.  In  Hering's  furnace  only  a  small  constant  fractional 
part  of  the  molten  metal  is  heated  in  the  resistor  channels,  but  on 
account  of  the  very  rapid  circulation,  the  heat  is  quickly  conveyed 
into  the  body  of  metal  in  the  crucible,  and  the  metal  in  the  channels 
does  not  become  overheated;  the  heating  of  such  a  furnace  may 
therefore  be  forced  to  advantage.  This  furnace  should  be  more  effi- 
cient than  Gin's  or  Kjellin's  furnaces,  as  it  has  a  much  smaller  sur- 
face for  radiating  heat,  and  the  walls  can  be  made  very  thick.  In  the 
use  of  this  furnace  for  steel-making,  the  systematic  circulation  rapidly 
exposes  fresh  surfaces  to  the  action  of  the  slag.  This  facilitates 
refining,  and  sulphur  is  very  quickly  taken  out  of  the  steel.  It  might 
be  supposed  that  the  resistor-holes  would  wear  away  rapidly  on  ac- 
count of  the  high  temperature  and  the  rapid  movement  of  their  con- 
tents, but  Dr.  Bering  informs  the  author  that  he  has  obtained  a  lin- 
ing material  which  is  very  satisfactory  and  can  readily  be  renewed  if 
necessary;  as  yet  the  holes  have  shown  no  signs  of  wear.  The  whole 
furnace  is  extremely  simple  in  construction.  Enough  metal  would 
usually  be  left  in  the  furnace  to  complete  the  electrical  connection 
between  the  two  resistor-holes,  and  when  this  is  not  the  case  a  fur- 
nace can  be  started  by  pouring  in  a  small  charge  of  molten  metal  to 
establish  the  electrical  connection.  The  furnace  is  used  for  melting 
and  refining  metals,  and  also  for  heating  any  substance  that  can  be 
floated  on  a  bath  of  some  inert  molten  metal. 

III.  Electrolytic  Furnaces 

In  these  furnaces  the  power  of  a  continuous  current  to  divide  a 
fused  chemical  compound  into  two  component  parts  is  utilized,  while 
the  heating  effect  of  the  current  is  also  needed  to  keep  the  contents  of 


DESCRIPTION  AND  CLASSIFICATION 


37 


the  furnace  in  a  state  of  fusion.  Most  chemical  compounds  can  be 
decomposed  in  this  way,  but  some  behave  like  the  metals  and  alloys, 
and  carry  the  current  without  suffering  decomposition.  Mixtures  of 
two  or  more  compounds  are  often  employed,  as  this  facilitates  the 
passage  of  the  current  and  renders  the  charge  more  fusible. 

Fig.  22  represents  a  furnace  for  the  electrolysis  of  fused  zinc 
chloride:  it  consists  of  a  chamber,  A,  containing  the  fused  chloride, 
B.  The  positive  electrode,  C,  is  made  of  carbon,  and  dips  into  the 
electrolyte,  while  the  fused  zinc,  D,  resulting  from  the  operation, 


FIG.  22. — Electrolytic  furnace. 


forms  the  negative  electrode;  electrical  connection  being  made  with 
it  at  E.  The  passage  of  the  current  splits  the  zinc  chloride  into  zinc, 
which  collects  at  D}  and  chlorine,  which  is  liberated  at  the  electrode, 
C,  and  is  withdrawn  from  the  furnace  by  the  pipe,  F.  A  cylinder,  G, 
passes  through  the  roof  of  the  furnace  and  dips  into  the  fused  elec- 
trolyte, to  enable  fresh  chloride  to  be  added  without  allowing  the 
chlorine  to  escape. 

Furnaces  for  the  production  of  aluminium  (Figs.  5  and  154)  are  also 
electrolytic. 

The  classification  adopted  in  this  chapter  is  shown  diagramatically 
in  the  following  table  and  in  the  chart  of  electric  furnaces,  Fig  23. 
An  example  of  each  class  is  given  in  the  table  which  is  numbered  to 
correspond  with  the  chart. 


38 


THE  ELECTRIC  FURNACE 


TABLE  I.— CLASSIFICATION  OF  ELECTRIC  FURNACES 


Arc  Furnaces 


With 
special 
Resistor 


Independent  Arc  (Moissan's  furnace),     (i) 

Single  Arc  (Girod).     (2) 
Double  Arc  (Heroult).    (3) 


Direct  Heating  Arc 


Resistor  in  Walls  (electrical  tube  furnace).     (4) 
Resistor  in  Charge  (carborundum  furnace).  (5) 


Without 
special 
Resistor 


Solid  (Graphite  furnace).     (6) 
Melting  (Heroult 

smelting  furnace).     (7) 
Liquid  (de  Laval  furnace).     (8) 
Electrolytic  (aluminium  furnace).     (9) 


Without 
Electro- 
lysis 


Resisting 

contents 

of  furnace 


FIG.  23. — Chart  of  electric  furnaces. 


CHAPTER  III 

EFFICIENCY  OF  ELECTRIC  AND  OTHER  FURNACES,  AND 
RELATIVE  COST  OF  ELECTRICAL  AND  FUEL  HEAT 

Electricity  would  generally  be  preferable  to  fuel  for  producing 
heat,  if  it  were  not  that  the  cost  of  electrical  energy  is  almost  invari- 
ably greater,  and  usually  many  times  greater  than  that  of  an  equi- 
valent amount  of  fuel.  In  certain  operations,  such  as  the  produc- 
tion of  carborundum  or  graphite,  electricity  must  be  employed, 
because  a  sufficiently  high  temperature  cannot  be  obtained  by  the 
combustion  of  fuel.  In  other  operations,  such  as  the  production 
of  aluminium,  the  electrolytic  action  of  the  electric  current  is  essential 
to  the  process.  A  large  number  of  metallurgical  operations,  how- 
ever, were  carried  on  successfully  before  electric  smelting  was  thought 
of;  and  for  such  purposes  electricity  is  only  employed  when  its 
greater  efficiency  and  convenience  out- weigh  the  usually  greater 
cost.  It  has  recently  been  realized  that  electricity  can  sometimes 
economically  replace  coal  or  coke  as  a  heating  agent  in  operations 
such  as  the  smelting  of  zinc  or  even  iron-ores,  or  in  the  production 
of  steel. 

In  comparing  electricity  and  coal,  we  may  consider  how  much  heat 
each  will  produce,  or  how  much  electrical  energy  will  be  needed  to  pro- 
duce as  much  heat  as  i  Ib.  of  coal  would  yield  on  burning.  One  unit 
or  kilowatt-hour  of  electrical  energy  will  produce  3,415  B.T.U. 
(British  Thermal  Units),  of  heat,  and  i  Ib.  of  good  quality  coal  will 
produce  about  14,000  B.T.U.  Thus  4  kw.-hours  are  needed  to 
produce  as  much  heat  as  i  Ib.  of  coal. 

For  small  consumers,  buying  electrical  energy  for  lighting  at 
10  or  15  cents  a  unit,  and  coal  at  $6  or  $7  a  short  ton,  the  cost  of 
electrical  heat  would  be  one  or  two  hundred  times  that  of  coal- 
heat. 

As  a  year  consists  of  8,766  hours,  i  kw.  would  yield,  if  operated 
continuously  for  that  time,  nearly  30,000,000  B.T.U.,  or  one  elec- 
trical horse-power  year  would  yield  22,320,000  B.T.U.;  and  as 
i  long  ton  of  coal  will  produce  about  31,000,000  B.T.U.,  it  will 
be  seen  that  an  electrical  horse-power  year,  produces  about  25 
per  cent,  less  heat  than  a  ton  of  good  coal;  or  i  ton  of  coal  would 

39 


40  THE  ELECTRIC  FURNACE 

produce  as  much  heat  as  ij  E.H.P.  years.  If  an  electrical  horse- 
power year  could  be  purchased  for  $30  and  a  long  ton  of  coal  for  $4, 
the  cost  of  electrical  heat,  per  B.T.U.  would  be  ten  times  the  cost  of 
coal-heat. 

In  localities  where  water-power  can  be  cheaply  developed, 
and  where  transportation  charges  for  coal  and  coke  are  high,  it 
may  be  possible  to  produce  electrical  power  at  $10  or  less  per  elec- 
tric horse-power  year,  in  large  amounts  as  would  be  necessary  for 
electric  furnace  work,  and  coal  may  cost  $6  or  $8,  while  furnace 
coke  might  cost  even  more  than  that.  Under  such  conditions  the 
cost  of  electrical  heat  would  be  less  than  twice  that  of  coal  heat, 
and  would  approximate  to  the  cost  of  heat  furnished  by  good 
furnace  coke. 

It  might  appear  from  these  figures,  that  electrical  heating  could 
not  be  profitably  employed,  except  under  the  most  extreme  condi- 
tions of  cheap  power  and  dear  fuel,  but  it  should  be  remembered 
that  in  an  electric  furnace,  a  large  proportion  of  the  heat  supplied 
is  actually  utilized  in  heating,  the  materials  in  the  furnace,  while 
in  a  coal-fired  furnace  this  is  not  always  the  case,  and  often,  par- 
ticularly in  high-temperature  furnaces,  the  greater  part  of  the  heat 
is  wasted,  and  only  a  small  proportion  is  utilized. 

The  efficiency  of  a  furnace  may  be  determined  by  finding  what 
proportion  of  the  heating  power  of  the  coal  or  the  electrical  energy 
supplied,  is  actually  utilized  in  heating  the  contents  of  the  furnace. 
The  following  table1  gives  typical  efficiencies  for  a  number  of 
furnaces. 

•x 

TABLE  II.— NET  EFFICIENCIES  OF  FURNACES  USED  FOR  MELTING 

METALS 

Per  Cent. 

Crucible  steel  furnaces,  fired  with  coke 2-3 

Reverberatory  furnaces  for  melting  metals 10-15 

Regenerative  open-hearth  steel  furnaces 20-30 

Shaft  furnaces  (foundry  cupolas,  etc.) 3°~~5° 

Large  electrical  furnaces 60-85 

These  efficiencies  relate  to  the  melting  of  metals,  but  similar 
figures  would  be  obtained  for  the  same  furnaces  employed  in  smelt- 
ing ores.  In  the  crucible  steel  furnace  and  the  reverberatory 
furnace,  the  greater  part  of  the  heat  is  carried  away  in  the  escaping 
gases,  which  are  necessarily  extremely  hot;  and  in  the  crucible 

1  The  figures  are  taken  from  Prof.  J.  W.  Richards'  "Metallurgical  Calcula- 
tions," Part  i,  p.  89. 


EFFICIENCY  OF  ELECTRIC  FURNACES 


41 


furnace  the  loss  is  additionally  high  on  account  of  the  slow  trans- 
mission of  the  heat  to  the  steel  inside  the  crucible.  In  the  open-, 
hearth  furnace,  the  loss  of  heat  due  to  the  escaping  gases  is  very 
much  less  because  the  heat  they  contain  is  given  to  the  brickwork 
in  the  regenerators  or  checker  chambers,  and  returned  from  these 
to  the  furnace  by  the  incoming  gas  and  air.  In  shaft  furnaces 
the  heat  of  the  furnace  gases  is  largely  absorbed  by  the  solid  mate- 
rials in  the  upper  part  of  the  furnace,  and  by  them  returned  to  the 
zone  of  fusion.  When  metals  are  melted  in  the  electric  furnace,  no 
gases  need  be  produced,  and  thus  a  large  waste  of  heat  is  entirely 
avoided;  while  the  furnace  gases  produced  in  the  electric  smelting 
of  ores  are  very  much  less  in  amount  than  those  from  similar  coal- 
or  gas-fired  furnaces.  The  amount  of  air  that  passes  through  most 


Electric 
Furnaces 
I  I  I 
htfft 
rnaces 

pen  Hearth 
Furnaces 


10  20  50  40 

FIG.  24. — Losses  of  heat  in  melting  metals. 

furnaces,  in  excess  of  that  required  to  burn  the  fuel,  increases  the 
loss  of  heat  by  the  furnace  gases;  and  the  incomplete  combustion 
of  the  fuel  is  another  source  of  loss.  The  large  loss  of  heat  by  con- 
duction and  radiation  from  the  furnace,  is  common  to  fuel  and  elec- 
tric furnaces,  and  depends  mainly  upon  the  size  and  temperature 
of  the  furnace;  the  larger  furnaces  having,  of  course,  a  smaller  rela- 
tive loss. 

Fig.  24  has  been  arranged  to  show,  for  each  class  of  furnace,  the 
heat  equivalent  of  fuel  or  electrical  energy  needed  to  impart  unit 
quantity  of  heat  to  the  metal  to  be  melted.  The  black  areas  indi- 
cate the  loss  of  heat,  the  upper  edge  of  each  area  showing  the  mini- 
mum, and  the  lower  edge,  the  maximum  loss  for  each  class  of  fur- 
nace. The  diagram  also  shows  that  for  one  heat  equivalent  of 
power  supplied  to  an  electric  furnace;  a  shaft  furnace  would  require 
nearly  two;  an  open-hearth  furnace  three;  a  reverberatory  furnace 
six;  and  a  crucible  steel  furnace  30  heat  equivalents  of  fuel,  in  order 
to  melt  the  same  amount  of  metal. 


42  TEE  ELECTRIC  FURNACE 

If  these  numbers  are  used  to  multiply  the  cost  of  a  ton  of  the  coal  or 
coke  used,  assuming  it  to  be  of  about  14,000  B.T.U.,  the  resulting 
prices  may  be  compared  with  the  cost  of  ij  E.H.P.  years,  and  will 
give  an  idea  whether  coal  or  electrical  heating  would  be  cheaper  in 
any  particular  case.  Thus  in  making  crucible  steel  with  furnace  coke 
at  $5  and  the  electric  horse-power  year  at  $30,  the  coke  used  would 
cost  $5X30  =  1150,  and  the  electrical  energy  would  cost  $30X1^ 
=  $40,  thus  making  a  good  case  for  the  electrical  production  of  cru- 
cible steel.  In  the  case  of  the  open-hearth  furnace,  electrical 
energy  at  $10  an  electric  horse-power  year  would  cost  a  little  more 
than  coal  at  $4  a  ton,  while  it  would  correspond  with  coke  at  $6  to 
$7  a  ton  in  a  shaft  furnace.1 

These,  numbers  are  based  on  the  mean  of  the  figures  given  by 
Prof.  Richards  for  the  usual  efficiencies  of  certain  classes  of  furnaces; 
and  in  any  selected  case  it  would  be  desirable  to  have  the  efficiencies 
of  the  particular  electrical  and  fuel  furnaces  to  be  compared.  The 
incidental  expenses  connected  with  each  method  of  smelting  should 
also  be  considered. 

The  results  do,  nevertheless,  give  a  fair  idea  of  the  conditions 
under  which  electrical  heat  could  commercially  replace  fuel-heat. 
They  show  clearly,  that  in  the  production  of  crucible  steel,  electrical 
power  should  be  able  to  replace  coke  as  a  source  of  heat.  The  writer 
pointed  out,  more  than  nine  years  ago,2  that  the  production  of 
crucible  steel  in  the  electric  furnace  was  technically  and  financially 
possible,  and  plants  for  this  purpose  are  now  in  operation  or  con- 
struction in  Sweden,  Germany,  England,  the  United  States,  Canada, 
and  elsewhere. 

In  comparing  the  cost  of  electrical  and  fuel  heating,  it  has  been 
assumed  that  the  full  heat-value  was  obtained  from  the  electrical 
horse-power  year.  To  obtain  this,  it  would  be  necessary  for  the 
furnaces  to  be  operated  at  their  full  load  for  every  minute  of  the  year, 
and  any  shut-down,  or  any  period  during  which  a  smaller  amount  of 
power  was  being  utilized,  would  lessen  the  useful  effect,  without  any 
corresponding  reduction  in  the  amount  paid  for  the  power;  as  it  is 
bought  by  the  year,  and  not  by  the  total  kilowatt-hours  consumed. 
In  the  case  of  fuel-fired  furnaces,  a  shut-down,  or  a  period  of  reduced 
output,  will  increase  the  working  cost  per  ton  of  product;  but  not  to 

1  See  also  editorial  "Electric  Heat  versus  Heat  from  Fuel,"  Electrochemical 
Industry,  vol.  v,  p.  298. 

2  Stansfield,  The  Electrothermic  Production  of  Iron  and  Steel,  Trans.  Can.  Soc. 
of  Civil  Engineers,  vol.  xviii,  Part  i  (1904),  p.  72. 


EFFICIENCY  OF  ELECTRIC  FURNACES  43 

the  same  extent,  as  the  fuel  is  usually  bought  by  the  ton,  and  not  on 
some  assumed  standard  of  maximum  consumption.  The  time  during 
which  a  single  electric  furnace  is  shut  down  for  repairs  will  necessarily 
increase  decidedly  the  working  cost  of  electrical  energy;  but  when 
electric  smelting  has  become  well  established,  the  losses  in  this  way 
will  not  be  heavy.  In  the  regular  operation  of  an  electric  smelting 
plant,  there  will  be  few  accidental  shut-downs,  all  working  furnaces 
will  be  kept  at  a  steady  load,  and,  by  means  of  spare  furnaces,  the 
full  load  will  be  maintained  during  the  periodical  lay-off  of  each  fur- 
nace for  repairs. 

Having  now  considered,  in  a  general  manner,  the  efficiency  of 
furnaces  and  the  relative  costs  of  electrical  and  fuel  heating,  the 
method  of  calculating  these  efficiencies  may  be  discussed. 

THE  CALCULATION  OF  FURNACE  EFFICIENCIES1 

The  word  heat  is  used  popularly  in  two  senses;  thus  "  the  heat  of  a 
furnace,"  meaning  how  hot  the  furnace  is,  is  quite  distinct  from 
the  amount  of  heat  produced  in  the  furnace  per  minute,  or  the  amount 
of  heat  needed  to  turn  a  pound  of  ice  into  a  pound  of  water.  The 
first  use  is  really  a  quality  of  the  hot  body,  and  to  avoid  confusion  the 
word  temperature  should  be  used  in  such  cases,  while  the  word  heat 
should  be  restricted  to  the  second  case,  in  which  the  quantity  of  heat 
is  referred  to.  A  definite  quantity  of  heat  can  be  supplied  at  a  high 
or  a  low  temperature,  just  as  a  definite  quantity  of  air  can  be  supplied 
at  a  high  or  a  low  pressure;  and  the  addition  of  heat  to  a  body  raises 
the  temperature,  just  in  the  same  way  that  pumping  air  into  a 
receiver  raises  the  pressure. 

Temperatures  are  measured,  as  is  well  known,  by  thermometers 
or  pyrometers  (the  latter  for  high  temperatures),  and  the  scales  of 
these  instruments  are  based  upon  the  temperatures  of  melting  ice 
and  boiling  water,  these  being  o°  and  100°  on  the  Centigrade  scale, 
and  32°  and  212°  on  the  Fahrenheit  scale.  The  use  of  these  two 
scales  complicates  technical  literature,  since  the  Centigrade  is  mainly 
used  for  scientific  purposes,  while  the  Fahrenheit  is  mainly  used  for 
ordinary  affairs,  and  it  is  often  necessary  to  state  temperatures  on 
both  scales  in  order  to  be  generally  understood.  The  conversion 
from  one  scale  to  the  other  is  simple  if  it  is  remembered  that  the  tem- 
peratures o°C.  and  100°  C.  are  the  same  as32°F.  and  212°  F.,  and 

1  For  a  full  account,  with  examples,  of  the  calculation  of  furnace  efficiencies, 
see  Prof.  J.  W.  Richards'  "Metallurgical  Calculations,"  Parts  I,  II,  and  III. 


44  THE  ELECTRIC  FURNACE 

that  a  difference  of  temperature  of  5°  on  tfie  Centigrade  scale  corre- 
sponds to  a  difference  of  temperature  of  9°  on  the  Fahrenheit  scale; 
whence  F.°=i.8  C°+32;  and  C.°  =  5/9  (F.°-32). 

Heat  is  measured  in  several  different  units,  thus  further  compli- 
cating technical  writings,  most  of  these  units  representing  the  a  mount 
of  heat  needed  to  raise  the  temperature  of  unit  weight  of  water 
through  i°.  By  selecting  different  weights  of  water,  as  the  pound, 
gram,  or  kilogram,  and  different  temperature  scales,  it  is  easy  to  get 
six  or  eight  different  units  of  heat,  thus  entailing  a  large  amount  of 
trouble,  both  in  the  statement  of  amounts  of  heat  and  in  changing 
these  from  one  system  of  units  to  another. 

The  following  heat  units  are  usually  used: 
The  Gram-calorie.— ( i  cal.)— The  amount  of  heat  needed  to  raise 

the  temperature  of  i  grm.  of  water  i°  C.  (from  10°  C.  to  11°  C.)1 
The  Kilogram-Calorie. — (i  Cal.) — The  amount  of  heat  needed  to 

raise  the  temperature  of  i  kg.  of  water  i°  C. 
The  Pound-Calorie. — (i   Calb.) — The  amount  of  heat  needed  to 

raise  the  temperature  of  i  Ib.  of  water  i°  C. 
The  British  Thermal  Unit.— (i    B.T.U.)— The   amount   of  heat 

needed  to  raise  the  temperature  of  i  Ib.  of  water  i°  F.  (from  60°  F. 

to6i°F.y 
The  Evaporative  Unit. — The  amount  of  heat  needed  to  convert  i 

Ib.  of  water  at  212°  F.  into  steam  at  the  same  temperature  (at 

normal  atmospheric  pressure). 

The  following  are  the  relations  between  these  different  units: 

1  The  value  of  the  gram  calorie  depends  upon  the  specific  heat  of  water  between 
10°  C.  and  11°  C.  (J.  A.  Fleming,  Cantor  lecture,  Journ.  Roy.  Soc.  of  Arts,  lix, 
191 1,  p.  834),  and  that  of  the  B.T.U.  on  its  specific  heat  between  60°  F.  and  61° 
F.  Recent  determinations  (H.  T.  Barnes,  The  mechanical  equivalent  of  heat 
measured  by  electrical  means,  Int.  Elect.  Congress,  St.  Louis,  1904,  p.  65)  show 
that  the  specific  heat  at  10°  C.  is  about  0.2  per  cent,  greater  than  at  60°  F.  This 
would  mean  that  in  converting  pound  calories  into  B.T.U.  by  the  factor  9/5  an 
error  of  about  0.2  per  cent,  would  be  made.  Also  as  the  measurement  of  heat  is 
usually  conducted  between  15°  C.  and  20°  C.,  a  correction  will  have  to  be  made 
to  reduce  the  results  to  calories  as  measured  at  10°  C.  For  these  and  other 
reasons  it  has  been  suggested  that  the  calorie  should  be  based  on  the  mean  specific 
heat  of  water  from  o°  C.  to  100°  C.  This  value  is  practically  the  same  as  if 
measured  at  15°  C.,  or  at  60°  F.  The  mechanical  equivalent  of  i  grm.  calorie  of 
heat  measured  at  15°  C.  is  about  4.186  joules,  making  the  heat  value  of 
i  kw.-second,  0.239  Cal.,  or  0.527  Calb. 


EFFICIENCY  OF  ELECTRIC  FURNACES  45 

Kilogram-Calorie1  =1,000  Gram-calories. 

Pound-Calorie  =453-6  Gram-calories. 

British  Thermal  Unit  =  5/9  of  a  Pound-Calorie. 

British  Thermal  Unit  =252  Gram-calories. 

Evaporative  Unit  =967  British  Thermal  Units. 

The  gram-  and  kilogram-calories  are  the  most  convenient  for 
scientific  investigations,  but  in  cases  where  the  weights  are  given 
in  pounds  the  pound-calorie  or  the  B.T.U.  must  usually  be  employed. 

Rate  of  Heating. — The  rate  at  which  heat  is  produced,  or  supplied 
to  a  furnace,  may  be  stated  in  any  of  the  foregoing  heat  units  com- 
bined with  any  convenient  unit  of  time.  Thus  if  3  Ib.  of  coke  are 
burnt  per  minute,  and  if  the  coke  has  a  calorific  power  of  7,000 
centigrade  units  (that  is  gram-calories  per  gram  of  coal,  or  pound- 
calories  per  pound  of  coal),  the  rate  of  heat  production  will  be 
21,000  Calb.  per  minute.  The  unit  of  electrical  power  is  the  watt — 
that  is  the  power  of  i  ampere  flowing  through  the  resistance  of 
i  ohm,  and  this  produces  heat  at  the  rate  of  0.239  ca^  Per  second. 
In  electric-furnace  calculations  it  is  often  convenient  to  use  the  watt 
as  the  rate  of  heat  production  instead  of  the  more  cumbrous  units 
just  mentioned. 

i  watt          =o.  239  grm.-calorie  per  second, 
i  kilowatt    =0.239  kg.-calorie  per  second, 
i  kilowatt    =0.527  Ib.-calorie  per  second. 

Dr.  Carl  Hering2  has  pointed  out  that  great  complication  is  caused 
by  measuring  electrical  energy,  heat  energy  and  chemical  energy 
in  different  units,  and  that  it  would  be  very  desirable  that  all  forms 
of  energy  should  be  expressed  in  electrical  units.  He  recommends 
the  watt-hour  or  kilowatt-hour  as  the  practical  unit  for  the  measure- 
ment of  all  kinds  of  energy:  he  gives  the  following  conversion 
factors: 

POWER 

i  watt  =  0.238882  grm.-calorie  per  second, 

i  watt  =  0.0568776  B.T.U.  per  minute, 

i  kilowatt       =14.3329  kg.-calories  per  minute. 

ENERGY 

i  watt-hour    =  0.859975  kg.-calorie. 
i  watt-hour     =   3.41266  B.T.U. 

1  In  order  to  distinguish  between  the  kilogram  calorie  and  the  gram  calorie  it  is 
usual  to  use  a  C  for  the  first,  and  a  c  for  the  second,  thus  100  kg.  calories  would 
be  written  100  Cal,  and  100  grm.  calories  would  be  zoo  cal.     The  author  suggests 
the  contraction  Calb.,  for  pound-calories. 

2  Carl  Hering,  Trans.  Am.  Electrochem.  Soc.,  xxi,  1912,  p.  499. 


46  THE  ELECTRIC  FURNACE 

The  efficiency  of  a  furnace  is  the  ratio  between  the  amount  of 
heat  usefully  employed  in  the  furnace  and  the  heat  value  of  the 
fuel  or  electrical  energy  supplied:  thus,  if  100  Ib.  of  steel  can  be  melted 
in  a  crucible  furnace  by  the  use  of  150  Ib.  of  coke,  and  if  300  lb.- 
calories  are  needed  to  melt  i  Ib.  of  steel  (this  having  been  determined 
by  experiment),  and  if  i  Ib.  of  coke  can  furnish  7,200  Ib.-calories 
(found  by  experiment),  the  efficiency  of  the  furnace  can  at  once  be 
obtained. 

.          _  Weight  of  steel  X  heat  needed  to  melt  i  Ib.  steel. 
n  V     Weight  of  coke  X  heat  furnished  by  i  Ib.  coke. 

.  100  Ibs.  X  300    Calb. 

Ernciency  = ^- — — —         0  1U  =0.028  =  2.8  per  cent. 

150  Ibs.  X  7,200  Calb. 

The  statement  that  300  Ib.-calories  are  needed  to  melt  i  Ib.  of 
steel  means,  that  if  to  i  Ib.  of  cold  steel  there  could  be  added  300 
Ib.-calories  of  heat,  without  any  of  the  heat  being  lost,  the  steel 
would  be  heated  to  its  melting-point  and  melted.  It  is,  of  course, 
impossible  to  do  this,  but  by  pouring  some  molten  steel  into  a  vessel 
of  water,  and  noting  the  rise  of  temperature  of  the  water,  the  number 
of  calories  given  out  by  the  steel  in  cooling  can  be  determined, 
and  this  is  obviously  the  same  as  the  amount  of  heat  needed  to  melt 
the  steel.  The  number  of  calories  being  equal  to  the  product  of 
the  weight  of  water  and  its  rise  of  temperature,  corrections  being 
made  for  the  heat  absorbed  by  the  vessel  and  otherwise  lost  during 
the  experiment. 

The  amount  of  heat  needed  to  melt  i  Ib.  of  each  of  the  common 
metals,  and  the  temperatures  at  which  they  melt,  are  given  in  the 
following  table;  the  figures  have  all  been  obtained  by  experiment, 
with  the  exception  of  the  heat  of  fusion  of  wrought-iron,  which  has 
been  calculated: 

The  figures  in  the  last  three  columns  really  represent  the  amount 
of  heat  given  out  by  i  Ib.  of  the  metal  in  cooling  from  the  molten 
state  to  32°  F.  In  heating  the  metal  from  60°  or  70°  F.  rather  less 
heat  will  be  needed,  but  on  the  other  hand,  some  additional  heat 
will  be  required  in  order  that  the  metal  shall  be  thoroughly  melted, 
and  the  heat  actually  needed  to  heat  the  metal  to  a  casting  tempera- 
ture will  be  a  little  more  than  the  figures  in  the  table. 

The  amount  of  heat  that  can  be  produced  from  i  Ib.  of  coke, 
can  be  determined  by  burning  a  small  weighed  quantity  of  the  coke 
in  a  calorimeter;  which  is  an  instrument  for  measuring  the  amount 
of  heat  that  is  produced.  The  amount  of  heat  produced  by  unit 


EFFICIENCY  OF  ELECTRIC  FURNACES 


47 


TABLE  III.— MELTING    TEMPERATURES    OF    METALS,    AND    AMOUNTS    OF 
HEAT  REQUIRED  TO  MELT  THEM 


Metal 

Melting  temperature       |       Heat  to  melt  i  Ib. 

C. 

F. 

Calb.1 

B.T.U. 

Watt- 
hrs. 

Tin                       .... 

232° 
327° 
419° 

657° 
920° 
1083° 
io27°-ii35° 
noo°-i275° 
1425° 

IS03° 

450° 
620° 
786° 
1214° 
1688° 

1983° 
i88o°-2075° 

2OI2°-2327° 
26000          ' 

2737° 

28 
16 
68 

258 
130 
162 

245 
300 

343 

5i 
28 

122 
465 
234 
292 

441 
540 
6l7 

i5 

8 
36 
136 
69 
85 

129 
158 
181 

Lead  

Zinc              

Aluminium 

Brass  (65  %  copper)  .... 
Cooper2 

Cast  iron  3  (white)  
Cast  iron  (gray) 

Tool  steel  (i  %  carbon)  .  . 
Wrousrht-iron.  . 

weight  of  a  fuel,  is  known  as  its  calorific  power,  and  is  usually  meas- 
ured in  the  corresponding  heat  units;  that  is,  heat  units  containing 
the  same  unit  of  weight;  as,  for  example,  the  number  of  gram-cal- 
ories produced  by  i  grm.  of  fuel;  the  number  of  pound- calories  pro- 
duced by  i  Ib.  of  fuel,  or  the  number  of  B.T.U.  produced  by  i  Ib. 
of  fuel.  The  first  two  of  these  results  will  obviously  be  identical, 
and  may  be  called  the  Centigrade  calorific  power,  while  the  last 
result  will  be  9/5  times  as  large,  and  may  be  called  the  Fahrenheit 
calorific  power.  Thus  the  calorific  power  of  carbon  is  8,100  on  the 
Centigrade  scale,  and  14,580  on  the  Fahrenheit  scale,  meaning  that 
one  part  by  weight  of  carbon  would  give  out  as  much  heat,  if  com- 
pletely burnt,  as  would  raise  the  temperature  of  8,100  parts  of  water 
i°  C.,  or  14,580  parts  of  water  i°  F.,  so  the  result  is  the  same,  what- 
ever unit  of  weight  is  selected.  When,  however,  the  fuel  is  measured 
by  volume,  as  in  the  case  of  a  gas,  it  will  be  necessary  to  state  the 
calorific  power  as  so  many  B.T.U.  per  cubic  foot,  or  calories  per 
cubic  foot,  or  per  cubic  meter.  Calorific  powers  are  also  sometimes 
stated  in  evaporative  units,  thus  avoiding  the  use  of  either  scale 
of  temperature.  If  Dr.  Bering's  suggestion  is  adopted  we  shall 
have  to  state  calorific  power  in  kilowatt-hours  per  kilogram  or  cubic 
meter  of  the  fuel. 
In  many  furnaces  the  carbon  in  the  fuel  is  not  burnt  completely, 

1  These  figures  are  mainly  from  Richards'  "Metallurgical  Calculations." 

2  The  figure  for  copper,  1,083°  C.,  is  its  melting  temperature  when  protected 
from  oxidation,  by  a  cover  of  charcoal  for  example.     Oxidized  copper  melts  at 
i,p62°  C. 

3  Melting  temperatures  of  cast-iron  were  determined  by  Prof.  H.  M.  Howe, 
see  his  Metallurgical  Laboratory  Notes,  p.  125. 


48  THE  ELECTRIC  FURNACE 

and  it  then  has  a  smaller  effective  calorific  power.  The  complete 
combustion  of  carbon  produces  the  gas  CO2,  containing  two  atoms 
of  oxygen,  while  its  incomplete  combustion  produces  the  gas  CO, 
containing  only  one  atom  of  oxygen.  The  calorific  power  in  the 
latter  case  being  only  2,430  C.,  or  4,374  F.,  which  is  less  than  one- 
third  of  its  calorific  power  when  burnt  completely.  The  iron  blast- 
furnace furnishes  a  good  example  of  this  loss  of  heat  through  the 
imperfect  combustion  of  the  coke.  In  order  to  thoroughly  reduce 
the  iron  ore  to  metal  a  large  amount  of  coke  must  be  present  in  the 
furnace,  and  this  can  only  be  burnt  to  CO  in  the  lower  part  of  the 
furnace,  thus  obtaining  far  less  heat  from  the  same  weight  of  coke 
than  if  it  could  be  burnt  completely  to  CO2.  The  CO  produced 
in  the  lower  part  of  the  furnace  is,  however,  partly  utilized,  higher 
up,  for  the  reduction  of  the  iron-ore,  and  the  CO  that  finally  escapes 
from  the  furnace  is  employed  as  a  fuel  for  heating  the  blast  and  for 
raising  steam  or  driving  a  gas-engine. 

In  determining  the  calorific  power  of  a  fuel  in  a  calorimeter, 
the  aqueous  vapor  resulting  from  the  burning  of  any  hydrogen 
in  the  fuel,  and  any  moisture  and  " combined  water"  in  the  fuel, 
will  be  condensed  to  water;  and  its  latent  heat  of  condensation  will 
be  included  in  the  resulting  calorific  power.  When  the  fuel  is  burnt 
in  any  metallurgical  furnace,  the  furnace  gases  escape  at  too  high 
a  temperature  to  allow  of  the  condensation  of  the  vapor,  and  in 
calculating  furnace  efficiencies  a  calorific  power  should  be  used 
that  does  not  include  the  heat  of  condensation  of  the  water  vapor, 
since  this  heat  can  never  be  obtained  in  the  furnace.  The  observed 
calorific  power  should,  therefore,  be  corrected  by  subtracting  from 
it  the  heat  of  condensation  of  all  the  water  vapor  that  is  present  in 
the  fuel,  or  is  produced  by  its  combustion.  The  corrected  value 
has  been  called  the  metallurgical  or  practical  calorific  power,1 
or  the  "net"  calorific  power  and  should  be  used  instead  of  the 
"gross"  or  calorimeter  calorific  power,  in  the  case  of  all  furnaces 
from  which  the  water,  contained  in  the  furnace  gases,  escapes  in 
the  form  of  vapor. 

The  following  table  contains  the  metallurgical  calorific  powers 
of  some  of  the  commoner  fuels,  and  some  pure  substances.  The 
calorific  powers  of  fuels  cannot,  however,  be  stated  exactly,  as  they 
vary  considerably. 

The  figures  in  this  table  are,  in  many  cases,  lower  than  the  calorific 
powers  obtained  experimentally  in  a  calorimeter,  the  difference 

1  Prof.  J.  W.  Richards'  loc.  tit. 


EFFICIENCY  OF  ELECTRIC  FURNACES 


49 


being  the  correction  of  606.5  Ib.-calories  per  pound  of  water  in  the 
products  of  combustion;  this  amount  of  heat  being  needed  to  evapo- 
rate a  pound  of  water  at  o°  C.  In  calculating  furnace  efficiencies  by 
means  of  this  table,  the  furnace  will  thus  be  debited  with  the  sensible 
heat  carried  by  the  water  vapor  as  well  as  with  that  carried  by  the 
other  furnace  gases,  but  the  heat  of  condensation  of  the  water  vapor 
will  have  been  removed  from  the  balance  sheet. 

TABLE  IV.— CALORIFIC  POWERS 

(All  water  remaining  uncondensed) 


C. 

F. 

Calb.1 

B.T.U.2 

Carbon  (burnt  to  CO2),  per  Ib  
Carbon  (burnt  to  CO)   per  Ib 

8,100 
2,4.30 

14,580 
4,774 

Carbon  monoxide  per  Ib  

2,430 

4,374 

Carbon  monoxide  per  cu   ft                            ... 

IQI 

744 

Hydrogen  per  Ib               

29,030 

52,254 

Hydrogen  per  cu  ft 

163 

2Q3  .  <J 

Methane  (Marsh  gas,  CH-O,  per  cu.  ft  
Ethylene  (Olefiant  gas   C2H4)   per  cu.  ft 

537 
004 

966 

1,627 

Wood  (air  dried),  per  Ib  
Peat  (air  dried)   per  Ib                                  ... 

about  3,000 
3,000—  4,000 

about  5,400 
5,400—  7,200 

Charcoal  (5  to  10  per  cent,  moisture),  per  Ib.  .  . 
Oven  coke  per  Ib        ,                      

7,000-  7,500 
6,000—  7,400 

12,500-13,500 
12,400—13,300 

Anthracite,  per  Ib  

6,500-  7,500 

11,500-13,500 

Bituminous  coal,  per  Ib  
Fuel  oil  per  Ib 

7,000-  8,000 
o  soo—  ii  ooo 

12,500-14,500 
17,000—20  ooo 

Natural  gas,  per  cu.  ft  
Coal  gas  per  cu  ft 

460-     540 

TOO—            36O 

830-     970 

CCO—        6C.O 

Water  gas,  per  cu.  ft  
Producer  gas  per  cu  ft 

140-      1  80 
«rc—        no 

250-     320 
ioo—      160 

Electrical  energy,  per  kilo  watt-  hour3  

1,807 

3,415 

Electrical  energy,  per  E.H.P.  hour  
Electrical  energy,  per  E.H.P.  year  of  8,766 
hours. 

1,415 
12,400,000 

2,547 
22,320,000 

The  calorific  powers  of  the  pure  substances,  forming  the  first 
part  of  the  table,  will  serve  as  data  for  calculating  the  calorific 
power  of  a  gaseous  fuel  of  known  composition,  and  will  enable 
approximate  figures  to  be  obtained  for  solid  and  liquid  fuels.  The 

1  The  values  for  pure  substances  in  this  column  are  those  adopted  by  Prof. 
Richards. 

2  These  values  are  obtained  by  multiplying  the  figures  in  the  previous  column 
by  the  factor  9/5. 

3  The  values  for  electrical  energy  have  been  calculated  in  terms  of  the  specific 
heat  of  water  at  15°  C.,  i  kilowatt-second  being  0.239  CaL,  or  0.527  Calb. 


50  THE  ELECTRIC  FURNACE 

coal  and  other  solid  fuels  in  the  lower  part  of  the  table  are  sup- 
posed to  be  in  the  condition  in  which  they  would  naturally  occur: 
the  wood  being  air  dried,  and  containing  some  20  to  25  per  cent, 
of  moisture;  the  peat  also  air  dried  and  retaining  20  to  30  per  cent, 
of  moisture;  the  charcoal,  coke  and  coal  have  the  usual  amounts 
of  ash  and  moisture.  The  figures  given  for  coal  and  other  fuels 
will  not  cover  all  cases,  but  are  intended  to  represent  the  ordinary 
run  of  fuels.  The  calorific  powers  of  gases,  per  cubic  foot,  corre- 
spond to  dry  gas  at  32°  F.,  and  would  be  about  5  per  cent,  less  at 
60°  F.,  and  7  per  cent,  less  at  70°  F.  on  account  of  the  increase  in 
volume  of  the  gas:  the  presence  of  moisture  would  still  further 
decrease  the  calorific  power.  These  figures  refer  also  to  gases  under 
the  normal  barometric  pressure  of  760  mm.  The  calorific  power  of 
unit  volume  varies  with  changes  of  pressure;  being,  for  example, 
decidedly  less  at  places  of  great  elevation. 

By  the  aid  of  Tables  III  and  IV  it  will  be  easy  to  obtain,  approxi- 
mately, the  percentage  efficiency  of  any  furnace,  whether  fired  by 
solid,  liquid,  or  gaseous  fuel,  or  heated  electrically — if  it  is  employed 
for  heating  and  melting  metals,  and  if  the  amount  of  fuel  or  electrical 
energy  corresponding  to  the  melting  of  a  certain  weight  of  metal  is 
known.  It  will  not  be  possible,  however,  to  calculate  in  the  same 
manner  the  efficiency  of  a  furnace,  such  as  an  open-hearth  steel  fur- 
nace, in  which  the  metal  is  kept  molten  for  some  hours  in  order  to 
allow  of  certain  changes  being  made  in  its  composition.  In  such  a 
furnace  the  efficiency  can  only  be  calculated  in  reference  to  the  time 
during  which  the  charge  was  being  heated.  During  the  remainder 
of  the  "heat"  the  furnace  may  remain  for  considerable  periods  with- 
out any  marked  rise  of  temperature,  although  fuel  is  constantly  being 
used;  thus  making  the  calculated  efficiency  zero  during  such  periods. 

The  efficiencies  of  metal-melting  furnaces  were  considered  first  on 
account  of  the  simplicity  of  the  calculation.  But  it  is  equally  pos- 
sible to  calculate  the  efficiency  of  a  blast  furnace,  or  an  electrical  ore- 
smelting  furnace,  in  which  the  heat  is  used,  not  merely  in  melting  a 
metal,  but  also  in  effecting  the  chemical  work  of  reducing  the  ore  to 
a  metallic  condition.  The  amounts  of  heat  necessary  for  the  forma- 
tion of  a  large  number  of  chemical  compounds  are  known,  and  by 
means  of  these,  it  is  possible  to  draw  up  a  balance  sheet  showing  what 
amount  of  heat  is  needed  for  the  chemical  reactions,  as  well  as  for 
melting  the  metal  and  slag  in  the  furnace.  The  efficiency  can  then 
be  calculated  as  in  the  simpler  cases. 

As  an  example  we  may  calculate  the  efficiency  of  a  Heroult  elec- 


EFFICIENCY  OF  ELECTRIC  FURNACES  51 

trical  steel  furnace,  operated  at  La  Praz,  France,  for  the  Haanel 
commission  in  March,  1904. 1  The  furnace — basic  lined,  was  making 
steel  by  melting  scrap  with  ore  and  lime. 

The  charge  selected  for  calculation  (number  660)  consisted  of: 

Steel-scrap 5,733  lb. 

Iron  ore 430  lb. 

Lime 346  lb. 

Other  additions  were  made  after  the  charge  was  melted,  but  for 
obtaining  the  melting  efficiency  it  will  only  be  necessary  to  consider 
the  operation  of  melting  this  charge  in  the  furnace. 

The  scrap  charged  had  the  following  composition: 

Carbon o.  no  per  cent.        Phosphorus 0.220  per  cent. 

Silicon 0.152  per  cent.        Manganese o.  130  per  cent. 

Sulphur °-°5S  Per  cent.        Arsenic. 0.089  Per  cent. 

Supposing  that  the  iron-ore  in  the  charge  contained  400  lb.  of 
ferric  oxide,  it  may  be  assumed,  that  during  the  melting  of  the  charge, 
this  was  reduced  to  ferrous  oxide  by  the  oxidation  of  most  of  the  metal- 
loids and  some  of  the  iron  in  the  original  scrap.  A  rough  calcula- 
tion shows  that  the  melted  charge  would  consist  of  about  5,660  lb.  of 
"dead  soft"  steel,  and  850  lb.  of  slag  rich  in  ferrous  oxide  and  lime, 
and  that  the  reaction  would  produce  some  24,000  Ib.-calories,  which 
makes  a  small  addition  to  the  heat  furnished  by  the  electric  current. 

Assuming  that  a  temperature  of  1,520°  C.  is  necessary  for  a  com- 
plete fusion  of  the  charge  (see  Table  III),  about  344  Calb.  will  be 
needed  to  melt  each  pound  of  soft  steel,  or,  in  all,  344X5,660  = 
1,947,000  Calb. 

The  slag  will  need  about  600  Calb.  per  pound  in  order  to  melt  and 
heat  it  to  the  same  temperature,  or,  in  all,  600X850  =  510,000  Calb. 

The  electrical  power  employed  was  215  kw.  during  the  first  hour, 
and  342  during  the  remainder  of  the  run;  the  current  being  supplied 
at  about  no  volts.  The  time  occupied  in  melting  the  charge  was 
5  1/3  hours,  and  the  electrical  energy  supplied  to  the  furnace  during 
this  time  was  1,680  kw.-hours. 

The  heat  supplied  by  the  electric  current  was: 
1,680X1,897=3,187,000  Calb.     (See  Table  IV.) 

In  the  operation  of  melting  the  charge  the  heat  utilized  may  be 
taken  as  that  needed  to  melt  the  steel  and  the  slag,  while  the  heat 

1  Report  of  the  Commission  appointed  to  investigate  the  different  electro- 
thermic  processes  for  the  smelting  of  iron-ores  and  the  making  of  steel  in  Europe, 
PP-  54,  55,  7i  and  72. 


52  THE  ELECTRIC  FURNACE 

supplied  to  the  furnace  is  produced  in  part  by  the  electric  current, 
and  in  part  by  the  reaction  between  the  scrap  and  the  iron- ore. 

BALANCE  SHEET  OF  HEAT 

Heat  supplied  to  the  furnace:  Calb. 

i, 680  kw.-hours  of  electrical  energy 3,187,000 

Reaction  between  steel  scrap  and  iron-ore 24,000 

Total 3,211,000 


Heat  utilized  in  the  furnace: 

To  melt  5,660  Ib.  of  soft  steel 1,947,000 

To  melt  850  Ib.  of  basic  slag 510,000 


Total 2,457,000 


Efficiency  of  furnace  =  '       =0.765  =  76.5  per  cent. 

-In  making  this  calculation  it  has  been  assumed  that  no  oxidation 
of  the  steel  scrap  took  place  except  by  reaction  with  the  iron-ore  in 
the  charge.  Such  an  assumption  would  be  quite  wrong  in  regard 
to  an  open-hearth  furnace,  where  the  flame  of  burning  gases  con- 
stantly plays  over  the  charge,  but  in  the  electric  furnace  the  charge  is 
largely  protected  from  the  air,  and  there  is  consequently  less  oxidation. 
If  any  considerable  amount  of  iron  were  burnt  in  this  way,  the  heat 
produced  by  its  oxidation  should  have  been  added,  in  the  balance 
sheet,  to  the  heat  supplied  to  the  furnace;  and  this  would  lower  the 
resulting  figure  for  the  efficiency. 

After  the  charge  was  completely  melted,  the  slag  was  poured  off, 
and  the  steel  further  purified  by  the  addition  of  fresh  slags,  made  of 
lime,  sand  and  fluor  spar.  After  these  were  removed,  the  steel  was 
recarburized  in  the  furnace  by  additions  of  "carburite"  (a  mixture 
of  iron  and  carbon)  and  ferro-silicon;  some  ferro-manganese  was  also 
added,  and  a  little  aluminium  in  the  ladle. 

The  yield  of  ingots  was  5,161  Ib.  of  tool  steel  of  the  following  com- 
position: 

Carbon i  .016  per  cent.        Phosphorus 0.009  Per  cent. 

Silicon o.  103  per  cent.        Manganese o.  150  per  cent. 

Sulphur 0.020  per  cent.        Arsenic 0.060  per  cent. 

Three  hours  were  required  for  the  purification  and  carburization 
of  the  steel,  making  a  total  of  8  1/3  hours,  and  a  total  consumption  of 
2,580  kw.-hours,  or  0.171  E.H.P.  years  per  ton  of  steel  ingots.  At 


EFFICIENCY  OF  ELECTRIC  FURNACES  53 

$10  per  E.H.P.  year,  the  cost  of  electrical  energy  for  the  ton  (2,240 
Ib.)  of  tool  steel  would  be  $1.71. 

COST  OF  ELECTRICAL  ENERGY 

Electrical  energy  for  smelting  is  usually  obtained  from  water- 
power,  as  this  is  generally  less  costly  than  steam  or  gas-engine  power. 
A  few  figures  may  be  quoted: 

The  Hydro- electric  Power  Commission  of  Ontario1  pays  $9.40 
per  year  for  a  continuous  electrical  horse-power  from  Niagara  Falls, 
if  the  amount  taken  exceeds  25,000  h.p.,  the  charge  being  made  on 
the  maximum  demand  or  "peak  load."  The  power  is  delivered 
to  the  Commission  at  Niagara  Falls  at  12,000  volts,  and  is  trans- 
mitted at  110,000  volts  to  various  cities  in  southwestern  Ontario. 
The  charges  made  to  these  cities  vary  with  the  distance  and  the 
amount  taken,  but  range  in  many  cases  from  about  $18  to  $26  per 
continuous  E.H.P.  year,  based  on  the  maximum  demand. 

The  Commission  delivers  power  from  other  sources  to  cities  in 
eastern  Ontario  at  costs  which  vary,  in  several  instances,  between 
$14  and  $23  per  E.H.P.  year,  and  which  will  fall  to  about  $12  to  $20 
when  the  present  demand  has  been  doubled.2 

In  Sweden  and  Norway  electrical  power  for  smelting  can  be  gen- 
erated at  as  low  a  figure  as  from  $4  to  $8  yearly.3 

In  parts  of  British  Columbia  the  cost  of  an  electrical  horse-power 
year  will  vary  from  about  $20  to  $40. 

All  the  above  charges  are  made  on  the  maximum  demand  and 
not  on  the  average  power  employed,  but  the  demand  for  power  in 
electric  smelting  will  be  far  more  steady  than  the  usual  industrial 
requirements  for  light  and  power.  The  charges  apply,  however,  to 
high-voltage  power  which  must  be  transformed  to  the  lower  voltages 
suitable  for  electric  furnaces. 

1  "Water  Powers  of  Canada,"  L.  B.  Denis  and  A.  V.  White,  Commission  of 
Conservation,  Ottawa,  1911. 

2  The  larger  prices  paid  by  some  cities  have  been  omitted  as  electric  smelting 
would  be  carried  on  near  a  source  of  power. 

3  Prof.  C.  E.  Lucke  (Electrochemical  Industry,  vol.  v,  p.  230,  June,  1907) 
gives  the  cost  of  water-power  as  $8.50  to  $25  per  kilowatt-year.     Dr.  R.  S. 
Hutton  (Electrochemical  Industry,  vol.  v,  p.  24,  January,  1907)  gives  figures  for 
cheap  water-power,  varying  from  $20  per  horse-power  year  at  Niagara,  to  $3 
per  horse-power  year  in  Norway.     Dr.  Haanel  (European  Report,  1904,  p.  32) 
says  he  is  "credibly  informed  that  the  water-power  at  Chats  Falls  can  be  devel- 
oped at  a  cost  to  produce  an  E.H.P.  year  at  the  rate  of  $4.50." 


54  THE  ELECTRIC  FURNACE 

Electrical  energy  derived  from  steam  power  is  usually  decidedly 
more  costly  than  from  water-power,  and  may  be  expected  to  cost, 
under  usual  conditions,  some  $60  to  $90  per  continuous  E.H.P. 
year  assuming  a  load  factor  of  80  per  cent,  or  90  per  cent.  If, 
however,  a  very  large  steam  plant  is  erected  for  continuous  use  in 
electric  smelting,  and  if  coal  can  be  purchased  cheaply,  the  cost 
might  be  reduced  very  greatly.  Mr.  W.  Sykes,1  states  that  in  the 
Pittsburg  district  a  25,000  kw.  power  station,  with  5,000  to  6,000 
kw.  generator  sets,  could  be  installed  at  a  cost,  of  $50  per  kilowatt, 
and  that  with  coal  costing  $1.25  per  ton,  the  cost  per  kilowatt-hour 
would  be  0.25  cents  at  80  per  cent,  load  factor,  0.3  cents  at  60  per 
cent,  load  factor,  and  0.4  cents  at  40  per  cent,  load  factor.  These 
figures  corresponding  to  $16.35,  $19-62  and  $26.16  respectively 
per  continuous  E.H.P.  year  of  8,766  hours.  Under  these  conditions 
electrical  power  obtained  from  steam  would  compete  in  cost  with 
hydro- electric  power. 

An  approximate  figure  for  the  cost  of  operating  a  large  steam- 
electric  station  can  be  had  by  charging  $15  yearly  for  each  kilowatt, 
and  adding  the  cost  of  the  coal,  allowing  2  Ib.  for  each  kilowatt-hour. 

A  large  amount  of  power  can  also  be  produced  by  means  of  gas 
engines  run  by  the  surplus  gas  from  iron  blast-furnaces.  A  furnace 
making  400  tons  of  pig-iron  daily  will  yield  enough  gas  to  produce 
10,000  h.p.  in  addition  to  heating  the  blast  and  running  the  blowing 
engine.  The  cost  of  running  a  large  electric  power-station  operated 
by  gas  engines,  in  England,  has  been  stated2  to  be  $12  per  E.H.P. 
year,  apart  from  the  amount  charged  for  the  gas. 

1  W.  Sykes,  "Power  Supply  to  Electric  Furnaces  for  Refining  Iron  and  Steel," 
Trans.  Am.  Electrochem.  Soc.,  xxi,  1912,  p.  383. 

2  B.  H.  Thwaite,  "The  Economic  Distribution  of  Electric  Power  from  Blast- 
furnaces," Journ.  Iron  and  Steel  Tnst.,  1907,  iii,  p.  190. 


CHAPTER  IV 
CONSTRUCTION  AND  DESIGN 

An  electric  furnace  consists  essentially  of  some  substance  R 
(Fig.  25),  through  which  an  electric  current  flows,  and  of  an  envelope 
C,  which  retains  the  heat  and  the  contents  of  the  furnace.  Carbon 
electrodes,  A  and  B,  are  usually  needed  to  convey  the  current  in  and 
out  of  the  furnace.  If  the  envelope  could  be  made  perfectly  heat- 
tight,  and  if  no  fresh  charge  were  introduced  during  the  operation, 
it  would  be  possible  to  obtain  any  temperature  in  R  up  to  the 
volatilizing-point  of  the  contents  of  the  furnace,  with  the  smallest 
electric  current,  provided  it  were  allowed  to  pass  for  a  sufficient 


FIG.  25. — Ideal  electric  furnace. 

length  of  time.  With  the  materials  actually  available  for  furnace 
construction  this  is  not  possible.  For  a  definite  size  and  construction 
of  furnace,  a  definite  rate  of  heat  production  will  be  needed  in  order 
to  attain  any  particular  temperature. 

The  rate  of  production  of  heat  is  measured  by  the  number  of 
watts  of  electrical  power  supplied  to  the  furnace,  and  may  con- 
veniently be  stated  in  watts  per  cubic  inch,  or  kilowatts  per  cubic 
foot  of  the  interior  volume  of  the  furnace.  The  rate  of  heat  pro- 
duction which  is  necessary  to  enable  a  certain  temperature  to  be 
attained,  may  be  calculated  from  a  consideration  of  the  area,  thick- 
ness and  conductivity  for  heat  of  the  walls  of  the  furnace; 
but  it  is  more  easily  obtained  by  reference  to  furnaces  of  similar  con- 
struction which  have  attained  definite  temperatures  with  a  definite 
electric  power. 

55 


56  THE  ELECTRIC  FURNACE 

The  above  considerations  apply  more  particularly  to  an  inter- 
mittent furnace,  such  as  the  Stassano  furnace,  in  which  a  charge 
of  ore  or  metal  is  submitted  to  the  heat  of  the  electric  current  until 
it  has  all  been  reduced  or  melted,  and  the  whole  of  the  furnace 
and  its  contents  has  been  heated  to  a  uniform  high  temperature. 
In  the  case  of  a  continuous  furnace,  such  as  the  Heroult  furnace 
employed  to  smelt  iron-ores  at  Sault  Ste.  Marie,  Fig.  78,  a  con- 
stant stream  of  cold  material  enters  the  furnace,  and  after  reduc- 
tion and  fusion,  is  tapped  out  as  molten  pig  and  slag;  only  a  portion 
of  the  contents  of  the  furnace  being  heated  at  any  one  time  to  the 
smelting  temperature.  In  such  a  furnace  the  temperature  attain- 
able is  limited  by  the  melting  temperature  of  the  charge;  any  in- 
crease in  the  rate  of  heat  supply  will  serve  mainly  to  increase  the 
rate  of  smelting,  without  materially  increasing  the  temperature  of 
the  furnace.  It  is  like  melting  ice  in  a  pail,  the  ice  melts  faster  on 
a  hot  day  than  on  a  cool  one,  but  the  water  surrounding  the  ice 
will  not  become  warm  as  long  as  there  is  any  ice  left  to  melt.  Even 
in  such  a  furnace  each  portion  of  the  charge  must  ultimately  be 
heated  to  the  smelting  temperature,  and  a  definite  rate  of  heat 
supply  is  needed  if  the  furnace  is  to  smelt  at  all. 

MATERIALS  OF  FURNACE  CONSTRUCTION 

The  materials  for  constructing  the  interior  of  electric  and  other 
furnaces,  should  be  infusible  at  the  temperature  of  the  furnace; 
should  resist  the  action  of  the  metallic  slags  or  other  contents  of  the 
furnace;  should  retain  the  heat  of  the  furnace  as  far  as  possible, 
and  should  be  capable  of  being  formed  into  bricks,  or  coherent 
linings,  that  will  resist  the  mechanical  action  of  the  charge  in  the 
furnace.  The  following  are  a  number  of  the  more  important 
materials  that  can  be  employed. 

Fire-clay  Bricks.1 — The  clay  from  which  these  are  made  consists 

1  Notes  on  the  New  Jersey  Fire-brick  Industry,  H.  Ries.  Amer.  Inst.  Mining 
Engineers,  vol.  xxxiv  (1904),  p.  254. 

Refractoriness  of  Some  American  Fire-brick.  R.  F.  Weber,  Amer.  Inst. 
Mining  Engineers,  vol.  xxxv,  p.  637. 

The  Fire-clays  of  Missouri.  H.  A.  Wheeler,  Amer.  Inst.  Mining  Engineers, 
vol.  xxxv,  p.  720. 

Determination  of  the  Refractoriness  of  Fire-clays.  H.  O.  Hofman  &  C.  D. 
Demond,  Amer.  Inst.  Mining  Engineers,  vol.  xxiv,  p.  42;  vol.  xxv,  p.  3;  vol. 
xxviii,  p.  435. 

"Clays,  their  Occurrence,  Properties  and  Uses."     H.  Ries,  1912. 

A.  H.  Sexton,  "Fuel  and  Refractory  Materials"  (text-book). 


CONSTRUCTION  AND  DESIGN  57 


of  pure  clay,  or  kaolin,  (A^Oa,  2SiO2,  2H2O),  with  a  variable  propor- 
tion of  silica  in  addition  to  the  amount  present  in  the  kaolin,  and  as 
little  as  possible  of  fluxing  materials  such  as  iron  oxide,  lime,  mag- 
nesia, potash  or  soda.  Even  silica  lowers  the  melting-point,  and 
should  be  present  only  in  moderate  amount.  These  bricks  are 
largely  used  for  lining  ordinary  metallurgical  furnaces,  but  are  not 
usually  sufficiently  refractory  for  electric  furnaces;  they  can,  how- 
ever, be  used  as  a  backing  for  more  refractory  material.  Being 
silicious  in  composition,  they  are  easily  fluxed  by  slags  containing 
metallic  oxides.  When  not  exposed  to  such  slags  they  will  stand 
temperatures  nearly  up  to  their  melting-point,  which  varies  for 
good  fire-clay  from  about  1,600°  C.  to  1,730°  C.  or  from  2,900°  F.  to 
3,150°  F.  They  should  be  laid  in  fire-clay  mud,  instead  of  lime 
mortar,  as  the  latter  would  crumble  away  if  strongly  heated,  and 
at  still  higher  temperatures  would  flux  the  bricks.  Fire-clay  bricks 
are  subject  to  a  considerable  shrinkage  when  fired.  This  shrinkage 
is  permanent  and  varies  in  amount  with  the  temperature  to  which 
the  bricks  have  been  heated.  Subsequent  heating  and  cooling,  at 
lower  temperatures,  causes  a  small  temporary  expansion  and  con- 
traction of  the  brick. 
The  following  is  an  analysis  of  a  good  American  fire-clay  brick:1 

Silica  ............    53  .  5  per  cent.      Lime  ............     0.5  per  cent. 

Alumina  .........   42.8  per  cent.      Magnesia  ........   o.  25  per  cent. 

Ferric  oxide  ......      1.5  per  cent.      Alkalies  ..........     0.6  per  cent. 

Silica  Bricks.  —  These  should  contain  about  95  per  cent,  to  97. 
per  cent,  of  silica,  SiC>2.  The  melting  temperature  of  silica  is  approxi- 
mately that  of  platinum,  being  about  1750°  C.,  or  3,180°  F.,2  and 
the  silica  brick  should  stand  up  to  about  1,700°  C.,  or  3,100°  F. 
They  are  useful  for  the  roof  and  other  parts  of  open-hearth  steel- 
furnaces,  that  are  exposed  to  a  very  high  temperature,  but  not 
'subjected  to  the  action  of  metallic  slags,  which  would  soon  flux 
them  away.  They  have  the  property  of  expanding  when  fired, 
and  their  expansion  and  contraction  when  subsequently  heated 
and  cooled  is  greater  than  that  of  fire-clay  bricks.  This  expansion 
of  silica  bricks  amounts  to  about  1/4  in.  per  foot  and  special  joints 
are  left  to  accommodate  it  when  building  a  furnace  roof  of  these 
bricks. 

1  Messrs.  Harbison-  Walker. 

2  C.  W.  Kanolt,  "Melting-points  of  Fire-bricks,"  Tech.  Paper  10,  Bureau  of 
Standards,  Washington,  1912;  Met.  and  Chem.  Eng.,  x,  1912,  p.  692;  Trans. 
Am.  Electrochem.  Soc.,  xxii,  1912,  p.  95. 


58  THE  ELECTRIC  FURNACE 

Silica  bricks  should  be  laid  in  a  silicious  mud  for  mortar,  and  in 
general,  refractory  bricks  should  be  laid  in  mortar  of  the  same 
composition  as  the  brick,  to  avoid  fluxing;  thus  it  would  not  do  to 
lay  basic  brick  in  silicious  mortar,  as  the  mortar  would  combine 
with  and  flux  part  of  the  brick. 

The  following  is  the  analysis  of  a  good  silica  brick  of  American 
manufacture: 

Silica 95-5  per  cent.      Lime 2.1  per  cent. 

Alumina 1.5  per  cent.      Magnesia o.  i  per  cent. 

Ferric  oxide 0.7  per  cent. 

Silica  Sand. — This  is  used  for  lining  the  hearth  of  the  "acid" 
open-hearth  furnace  and  also  for  the  hearth  of  some  reverberatory 
furnaces  for  smelting  copper  ores.  Natural  sands  are  often  used 
and  should  contain  enough  impurity  to  enable  them  to  frit  or  set, 
but  not  so  much  as  to  render  them  unduly  fusible.  The  following 
analysis  of  a  sand  suitable  for  lining  an  open-hearth  furnace  is 
quoted  by  Harbison- Walker: 

Silica 97-25  per  cent.        Alkalies ." 0.36  per  cent. 

Alumina  and  iron  oxide    o .  16  per  cent.        Water o .  24  per  cent. 

Lime 0.08  per  cent.        Loss  on  ignition 0.36  per  cent. 

Magnesia 0.39  per  cent. 

The  sand  is  spread  in  a  thin  layer  (about  1/2  in.)  over  the  bottom 
of  the  furnace,  and  is  then  heated  until  it  sets.  Another  layer 
js  then  added,  and  this  is  repeated  until  a  solid  bottom  of  silica 
has  been  obtained  of  the  required  thickness  and  shape. 

Canister.1 — This  is  a  clay-bearing  sandstone  found  in  the  vicinity 
of  Sheffield  (England)  and  elsewhere.  It  contains  nearly  90  per 
cent,  of  silica  and  about  10  per  cent,  of  alumina,  and  can  be  bonded 
by  means  of  the  clay  which  it  contains.  It  can  be  manufactured  into 
bricks,  which  are  somewhat  less  refractory  than  the  true  silica  bricks," 
but  it  is  largely  used  as  a  crushed  material  for  making  the  rammed 
linings  of  Bessemer  converters  and  other  furnaces.  Canister  is  made 
artificially  by  mixing  crushed  silica  with  enough  clay  to  make  it 
bind. 

Lime,  CaO. — This  is  an  extremely  refractory  material,  and  is  use- 
ful for  lining  small  electric  furnaces.  Its  melting  temperature  is 

1  In  the  United  States  of  America  the  word  "ganister"  has  a  different  meaning. 
It  is  applied  there  to  high-grade  silica  rock,  containing  over  95  per  cent,  of 
silica,  and  to  the  silica  bricks  made  by  bonding  such  rock  with  the  addition  of 
lime.  See  F.  T.  Havard,  Refractories  and  Furnaces. 


CONSTRUCTION  AND  DESIGN  59 

not  exactly  known,  but  may  be  about  2,050°  C.,  or  3,700°  F.1 
Lime  is  obtained  by  burning  limestone,  (CaO,  CO2),  thus  driving 
off  the  carbon  dioxide  which  it  contains.  Burnt  lime  absorbs  mois- 
ture from  the  air  and  slakes,  forming  the  hydroxide  CaO,  H20. 
Lime  mortar  contains  water  and  carbon  dioxide,  and  when  it  is 
heated  in  a  furnace,  these  are  driven  off,  and  the  mortar  crumbles 
away.  Lime  cannot  be  made  into  fire-bricks  by  mixing  it  with  water, 
as  the  bricks  would  crumble  in  the  furnace,  and  it  is  difficult  to  render 
lime  coherent  by  the  use  of  any  other  material.  This  difficulty  of 
binding  and  liability  to  slake  has  prevented  the  general  use  of  lime 
for  furnace  linings.  Small  electric  and  oxy-hydrogen  furnaces  may 
be  constructed  of  blocks  of  quick-lime  or  of  the  natural  limestone 
which  becomes  converted  internally  into  lime  during  the  operation 
of  the  furnace.  Being  basic  or  non-silicious  in  character,  lime  will 
resist  the  action  of  metallic  slags,  and  it  would  form  a  valuable 
material  for  lining  electric  and  other  furnaces  if  it  were  not  for  the 
objections  already  mentioned.  The  use  of  lime  in  the  electric 
furnace  is  also  limited  by  its  property  of  forming  a  fusible  carbide 
when  heated  with  carbon. 

Lime  that  has  been  fused  in  an  electric  furnace  is  very  compact, 
will  stand  heating  followed  by  sudden  cooling,  and  becomes  hydrated 
very  slowly  when  exposed  to  moist  air  or  placed  in  water.  It  may 
prove  to  be  of  value  as  a  refractory  material.2 

Magnesia. — Burnt  Magnesite,  "Magnesite"  Bricks,  MgO.  Mag- 
nesia is  even  more  refractory  than  lime,  melting  at  perhaps  2,200°  C., 
or  4,000°  F.  It  is  produced  by  burning  magnesite  (MgO,  CO2) 
thus  driving  off  the  carbon  dioxide,  in  the  same  way  that  lime  is 
produced  from  limestone.  Although  it  resembles  lime  chemically, 
magnesia  does  not  slake  very  easily,  and  when  strongly  burned  it 
shrinks  considerably,  forming  a  heavy  material  very  different  from 
the  light,  chemically  prepared  magnesia  which  is  used  as  a  medicine. 
The  shrunk  magnesia  can  be  cemented  together  to  form  a  moder- 
ately strong  fire-brick,  which  is  extremely  valuable  for  lining  basic 
open-hearth  furnaces  and  electric  furnaces.3  It  is  not  easily  fluxed 
by  metallic  slags,  since  it  is  basic  in  composition.  On  account  of 
their  great  compactness  (a  brick  weighs  about  8  1/2  lb.),  they  are 

1  Boudouard  apparently  assumes  the  melting  temperature  of  lime  to  be  about 
2,050°  C.  Journ.  Iron  and  Steel  Inst.,  1905,  i,  p.  353. 

2  F.  A.  J.  FitzGerald,  Refractories,  Met.  and  Chem.  Eng.,  x,  (1912),  p.  129. 

3  C.  W.  Kanolt,  loc.  cit.,  found  one  brand  of  magnesia  brick  to  melt  at  2,165° 
C.  and  pure  magnesia  would  be  somewhat  more  refractory. 


60  THE  ELECTRIC  FURNACE 

very  good  conductors  of  heat,  being  about  twice  as  good  as  fire-clay 
bricks,  and  in  constructing  electric  furnaces  of  "magnesite"  bricks  an 
outer  coating  of  some  other  material  should  be  used  to  diminish  the 
loss  of  heat,  except  when  this  cooling  is  desired  to  prevent  the  flux- 
ing of  the  walls.  Magnesite  bricks  are  liable  to  crack  under  the  influ- 
ence of  heat  unless  it  is  gradually  applied.  Their  property  of  con- 
tracting when  heated  renders  them  unsuitable  for  building  the  arched 
roofs  of  furnaces,  and  silica  bricks  would  be  used  for  this  purpose 
except  in  furnaces  where  the  roof  was  exposed  to  a  temperature  at 
which  they  would  melt. 

Furnace  linings  may  also  be  constructed  of  burnt  magnesite  in 
the  form  of  powder;  it  is  mixed  with  tar  or  pitch  to  make  it  bind, 
and  rammed  into  place  around  a  core  by  means  of  a  hot  iron-rod. 

In  making  the  bottoms  of  basic  open-hearth  furnaces,  the  burnt 
magnesite  is  mixed  with  a  small  proportion  of  open-hearth  slag  and 
applied  in  thin  layers  as  in  the  process  of  making  a  sand  bottom. 
'For  electric  furnaces  tar  and  pitch  would  be  preferred  as  a  bond, 
as  the  resulting  lining  would  be  less  fusible.  It  is  difficult,  espec- 
ially in  small  furnaces,  to  get  a  compact  lining  with  the  use  of  tar 
or  pitch  and  some  other  bond  such  as  clay,  or  boracic  acid,  may  be 
employed.  Magnesia  does  not  combine  with  carbon  to  form  a 
carbide,  and  on  this  account  its  use  in  the  electric  furnace  is  pref- 
erable to  that  of  lime. 

Austrian  magnesite1,  which  is  very  suitable  for  the  hearths  of 
basic  open-hearth  furnaces,  has  the  following  composition  when 
burnt: 

Magnesia 85 . 32  per  cent.    Silica 2 . 84  per  cent. 

Lime 1.12  per  cent.    Ferric  oxide 8.57  per  cent. 

Alumina 0.93  per  cent.    Carbon  dioxide o.  50  per  cent. 

Grecian  magnesite  is  purer  and  more  refractory  than  the  Aus- 
trian variety,  and  on  this  account  it  is  less  used  in  open-hearths, 
as  it  is  difficult  to  frit  it  in  place.  For  electric  furnaces,  where 
higher  temperatures  can  be  attained,  Grecian  magnesite  should 
prove  useful. 

The  shrinkage  and  consequent  cracking  of  magnesite  bricks  and 
furnace  linings  when  exposed  to  heat,  is  much  reduced  if  the  magnesite 
has  been  calcined  at  a  very  high  temperature,  in  an  electric  furnace, 
or  has  even  been  fused  electrically.  Such  magnesite  is  also  almost 
free  from  any  tendency  to  absorb  carbon  dioxide  from  the  air.  Elec- 

1  Harbison-Walker  Refractories  Co.    "  The  Open-hearth  Furnace  and  Process.', 


CONSTRUCTION  AND  DESIGN  61 

trically  fused  magnesia  forms  a  very  compact  and  refractory  material 
for  lining  electric  furnaces,  or  it  may  be  applied  as  a  paste  mixed 
with  silicate  of  soda  to  render  ordinary  fire-clay  bricks  more  refrac- 
tory.1 It  is  cheaper,  however,  and  equally  satisfactory,  to  cal- 
cine the  magnesia  in  a  resistance  furnace  instead  of  fusing  it.2 

Dolomite. — This  is  a  limestone  containing  a  considerable  pro- 
portion of  magnesite,  and  when  burnt  it  forms  a  valuable  refrac- 
tory material,  which,  like  burnt  magnesite,  may  be  employed  as 
a  powder  for  furnace  linings.  It  resembles  magnesite,  but  is  not 
quite  so  good.  Burnt  dolomite  is  liable  to  become  slaked  when  ex- 
posed to  the  air,  but  of  course  far  less  rapidly  than  burnt  lime. 
On  this  account  and  because  it  does  not  form  so  dense  and  vitreous 
a  bottom  as  magnesite,  open-hearth  bottoms  are  usually  made  of 
magnesite,  and  dolomite  is  only  used  as  a  cheaper  material  for 
patching. 

The  following  is  an  analysis  of  (unburnt)  dolomite  suitable  (when 
burnt)  for  this  use:3 

Magnesia 17.31  per  cent.      Alumina  and  iron. .    3 . 74  per  cent. 

Lime 29.  20  per  cent.      Loss  on  ignition.  .  .  43 . 82  per  cent. 

Silica 5.52  per  cent. 

Chromite,  or  chrome  iron-ore,  is  a  neutral  refractory  material 
(neither  acid  nor  basic)  and  has  a  very  high  melting-point  (about 
2,180°  C.  or  3,950°  F.)1  in  spite  of  the  considerable  percentage  of 
iron  which  it  contains.  Its  formula,  when  pure,  is  FeOCr2O3. 

An  analysis  of  chromite  quoted  by  Messrs.  Harbison-Walker 
is  as  follows: 

Chromium  sesqui  oxide,  Cr2O3 38-40  per  cent. 

Ferric  oxide,  Fe2O3 17.5  per  cent. 

Alumina,  A12O3 24. 5  per  cent. 

Silica,  SiO2 3 .  25  per  cent. 

Magnesia,  MgO 15.  per  cent. 

Chromite  is  used  both  in  lump  and  powder  for  lining  the  crucibles 
of  blast-furnaces  for  copper  smelting,  and  in  similar  places  where 
there  is  a  corrosive  slag  which  would  corrode  an  acid  or  basic 
lining. 

Chromite  can  be  bonded,  with  clay  or  other  binder,  to  make  fire- 

1  Electrically  shrunk  magnesia,  see  paper  by  E.  K.  Scott,  quoted  Electro- 
chemical Industry,  vol.  iii  (1905),  p.  140. 

2  FitzGerald,  "Refractories,"  Met.  and  Chem.  Eng.,  vol.  x  (1912),  p.  129. 

3  Harbison- Walker,  "A  Study  of  the  Open-hearth,"  1911. 


62      .  THE  ELECTRIC  FURNACE 

bricks.  These  are  used  in  the  open-hearth  furnace  at  points  which 
are  especially  exposed  to  the  action  of  the  flame.  The  melting-point 
of  a  chromite  brick  has  been  found  to  be  2,050°  C.  or  3,720°  F.1 

Although  extremely  refractory  under  oxidizing  conditions  such  as 
obtain  in  the  open-hearth  furnace,  the  use  of  chromite  and  chrome 
bricks  in  the  electric  furnace  is  limited  because  the  oxides  of  iron 
and  chromium  are  reduced  to  metal  when  strongly  heated  in  the 
presence  of  carbon. 

In  furnaces  constructed  partly  of  silica  bricks,  and  partly  of 
dolomite,  or  magnesite  bricks,  it  would  be  expected  that  they 
would  flux  one  another  at  the  line  of  contact.  On  this  account, 
a  course  of  chromite  brick  is  sometimes  introduced  as  a  parting 
layer  between  the  two,  as  this  brick,  itself  very  refractory,  does 
not  easily  flux  with  either  acid  (siliceous)  or  basic  materials.  When 
magnesite  bricks  are  used,  however,  it  is  found  that  this  precaution 
is  unnecessary. 

Bauxite  is  a  hydrous  oxide  of  alumina  containing  approximately:2 

Alumina 60  per  cent.     Silica 4-7  per  cent. 

Water 30  per  cent.     Ferric  oxide 3-5  per  cent. 

It  is  refractory,  melting  at  about  1,820°  C.  or  3,300°  F,  and  is 
plastic  like  clay.  It  cannot  be  used  for  furnace  linings  on  account 
of  its  great  shrinkage  at  furnace  temperatures.  When  formed  into 
bricks  it  constitutes  a  very  valuable  refractory  material  which  is 
suitable  for  lining  rotary  cement-kilns  and  lead-refining  furnaces 
where  they  are  exposed  to  corrosive  lead -slags.  Alumina,  the  main 
constituent  of  bauxite,  is  sometimes  regarded  as  a  base  and  some- 
times as  an  acid,  in  metallurgical  practice.  Bauxite  may  therefore 
be  considered  a  neutral  refractory  material  like  chromite.  The 
melting-point  of  pure  alumina  has  recently  been  found  to  be  2,010°  C. 
or  3,650°  F.3 

Alundum.4 — This  is  electrically  fused  alumina  made  by  purifying 
and  fusing  bauxite  in  the  electric  furnace.  It  is  used  as  an  abrasive 
but  is  also  a  valuable  refractory  material. 

There  are  two  forms,  white  and  brown.  The  white  is  the  purer, 
containing  less  than  i  per  cent,  of  impurities,  and  is  stated  to  melt 

1  C.  W.  Kanolt,  loc.  tit. 

2  Harbison-Walker,  loc.  cit. 

3  C.  W.  Kanolt,  loc.  cit. 

4  Alundum,  see  page  357.     Met.  and  Chem.  Eng.,  vol.  viii  (1910),  p.  290. 
Saunders  Am.  Electrochem.,  vol.  xix   (1911),   p.   333.     FitzGerald  Met.  and 
Chem.  Eng.,  vol.  x  (1912),  p.  129. 


CONSTRUCTION  AND  DESIGN  63 

between  2,050°  C.  and  2,100°  C.1  The  brown  variety  contains 
6  to  8  per  cent,  of  impurities  (oxides  of  iron,  silicon  and  titanium) 
and  melts  not  more  than  50°  C.  below  the  white  variety. 

The  crushed  alundum  powder  is  molded  with  the  addition  of  clay 
or  similar  material  into  bricks,  tubes,  muffles,  crucibles,  etc.  These 
products  are  very  refractory,  standing  at  least  1,950°  C.  before 
melting,  but  they  are  rather  easily  fluxed  by  slags,  and  are  porous, 
so  that  they  cannot  be  used  for  protecting  pyrometers  or  construct- 
ing gas-tight  furnaces.  The  thermal  conductivity  of  molded  alun- 
dum is  high,  being  about  twice  that  of  porcelain  and  three  or  four 
times  that  of  fire-clay.  This  renders  alundum  particularly  suitable 
for  the  construction  of  muffles,  and  for  the  tubes  of  electrical 
tube-furnaces. 

Alundum  bricks  have  been  tried  instead  of  silica  bricks  for  the 
roof  of  a  high-temperature  electrical  furnace  and  have  given  good 
service;  but  on  account  of  their  high  thermal-conductivity  the 
alundum  bricks  must  be  covered  with  a  layer  of  fire-bricks,  or 
other  refractory  material  of  low  conductivity,  in  order  to  prevent 
an  excessive  loss  of  heat.  Alundum  bricks  do  not  stand  very  well 
however  when  used  for  the  roof  of  an  electric  steel  furnace;  being 
destroyed  by  the  lime- vapor  rising  from  the  charge. 

Carbon. — (Coke,  Charcoal,  Graphite.)  Carbon  is  the  most  re- 
fractory substance  known;  it  has  never  been  melted,  but  softens 
and  volatilizes  at  the  temperature  of  the  electric  arc,  that  is  about 
3,600°  C.,  or  6,500°  F.2  In  its  more  compact  forms  it  is  a  fair 
conductor  of  electricity  and  of  heat,  the  former  quality  together 
with  its  infusibility  enabling  it  to  be  used  for  electrodes  to  lead  the 
current  into  electric  furnaces.  Being  combustible  it  is  liable  to 
waste  away  when  exposed  to  the  air  at  a  red  heat,  and  for  the  same 
reason  it  is  corroded  when  exposed  to  slags  that  contain  easily  re- 

1  These  figures  must  be  too  high  if  we  accept  Kanolt's  figure  of  2,010°  C.  for 
pure  alumina. 

2  The  temperature  of  the  positive  carbon  of  the  electric  arc  was  determined  by 
Violle  to  be  3,500°  C.,  and  he  modified  this  figure  later  to  3,600°  C.  (Wright, 
Electric  Furnaces,  p.  9).    Le  Chatelier  obtained  the  figure  4,100°  C.  by  his  optical 
pyrometer  (Le  Chatelier  &  Boudouard  High  Temperature  Measurements,  p. 
155).    Lummer,  by  a  radiation  method  gives  the  temperature  as  between  3,500° 
C.  and  3,900°  C.  (Le  Chatelier  &  Boudouard,  p.  212).     Fery  has  obtained  the 
values  3,490°  C.,  3,869°  C.,  and  3,897°  C.  by  different  optical  methods   (Wright, 
p.  277).     The  value  3,700°  C.  is  adopted  by  Richards  (Metallurgical  Calculations, 
vol.  i,  p.  62),  as  the  "boiling-point"  of  carbon.     G.  K.  Burgess  states  the  tem- 
perature as  3,600°  C.  ±  150°,  Met.  and  Chem.  Engng.,  x,  1912,  p.  692. 


64  THE  ELECTRIC  FURNACE 

ducible  metallic  oxides.  Carbon  exists  in  the  three  different  forms 
of  amorphous  carbon,  graphite  and  diamond;  charcoal,  coke  and 
the  other  common  forms  of  carbon  being  of  the  amorphous  variety. 
When  amorphous  carbon  or  the  diamond  are  heated  to  the  tem- 
perature of  the  electric  arc,  they  are  changed  into  graphite.  Car- 
bon blocks,  composed  of  coke  or  graphite,  can  be  used  for  lining 
furnaces,  provided  they  are  not  exposed  to  air  or  to  oxidizing  slags, 
but  carbon  has  not  been  much  used  for  metallurgical  furnace  lin- 
ings. In  the  electric  furnace  it  is  often  employed,  forming  a  lining 
which  also  serves  as  an  electrode,  as  in  the  Heroult  iron-smelting 
furnace,  Fig.  78,  the  aluminium  furnace,  Fig.  5,  and  the  Willson 
carbide  furnace,  Fig.  7;  but  it  cannot  usually  be  employed  for  the 
entire  lining,  because  it  is  so  good  a  conductor  of  electricity  that 
the  current  would  tend  to  be  short-circuited  by  the  lining  instead 
of  passing  through  the  charge  or  resistor  in  the  furnace.  Coke- 
powder  can  be  used  for  lining  parts  of  furnaces,  using  pitch  or  tar 
as  a  binder,  and  such  linings  will  conduct  the  electric  current  and 
may  be  used  as  electrodes.'  In  experimental  work  a  lining  of  char- 
coal-powder cemented  with  molasses  and  water  may  sometimes  be 
used,  and  has  the  advantage  that  it  retains  the  heat  of  the  furnace 
very  well,  and  being  a  poor  electrical  conductor,  it  can  be  used  for 
the  entire  lining  without  fear  of  short-circuiting  the  current.  If 
exposed  to  the  air,  however,  it  will  burn  up  completely  if  it  once 
reaches  a  red  heat.  Graphite  is  a  better  conductor  of  electricity 
and  of  heat  than  amorphous  carbon,  and  is  less  easily  oxidized  by 
air  or  metallic  slags;  hence,  electrodes  are  often  composed  of  it. 
Graphite  is  often  used  in  the  construction  of  crucibles,  the  graphite 
being  mixed  with  its  own  weight  of  fire-clay.  The  graphite  renders 
the  fire-clay  refractory,  and  the  fire-clay  protects  the  graphite 
from  oxidation.  These  crucibles  are  not  so  refractory  as  the  graph- 
ite alone,  and  for  electric-furnace  experiments,  crucibles  may  be  cut 
out  of  a  block  of  graphite  or  retort  carbon. 

For  lining  electric  furnaces,  when  carbon  is  undesirable,  some 
products  of  the  electric  furnace  itself  are  very  suitable.  They 
are  not  so  refractory  as  carbon,  but  are  more  refractory  than  the 
other  furnace  materials  such  as  magnesia,  silica,  lime  or  alumina. 

Carborundum.1 — -This  is  produced  by  heating  silica  and  carbon 

1  The  Carborundum  Furnace,  F.  A.  J.  FitzGerald,  Electrochemical  Industry, 
vol.  iv  (1906),  p.  53. 

The  Electrochemical  Industries  of  Niagara  Falls,  F.  A.  J.  FitzGerald,  Electro- 
chemical Industry,  vol.  iii,  p.  253. 


CONSTRUCTION  AND  DESIGN  65 

to  a  very  high  temperature  in  the  electric  furnace.  It  is  a  crystal- 
lized compound  of  silicon  and  carbon  having  the  formula  SiC,  and 
besides  being  valuable  as  an  abrasive,  it  forms  a  very  refractory 
furnace-lining.  The  carborundum  powder  can  be  made  to  cohere 
by  the  use  of  fire-clay  (6  parts  of  the  powder  to  i  of  fire-clay),  or 
by  a  solution  of  silicate  of  soda,  or  water  glass,  which  should  be  very 
dilute  if  the  highest  temperatures  are  to  be  reached,  as  the  silicate 
of  soda  makes  the  carborundum  less  refractory.  Tar  or  glue  can 
also  be  used  as  binding  materials,  and  a  very  strong  brick  can  be 
obtained  by  using  glue  as  a  temporary  cement  and  exposing  the 
molded  article  to  an  oxidizing  atmosphere  at  a  high  temperature 
for  some  hours,  when  the  partial  oxidation  of  the  carborundum 
furnishes  silica  which  acts  as  a  permanent  bond. 

Carborundum  Fire-sand. — This  is  a  name  applied  to  the  un- 
crystallized  variety  of  carborundum,  which  is  found  in  the  cooler 
parts  of  the  carborundum  furnace.  It  only  differs  from  carborun- 
dum in  not  being  crystallized,  and  can  be  used  in  the  same  manner 
as  a  refractory  material. 

Crystolon1  is  a  trade  name  given  to  the  carbide  of  silicon  manu- 
factured by  the  Norton  Company  and  refers  particularly  to  the 
crystalline  variety. 

Silicon  carbide  powder  may  be  bonded,  as  explained  above  under 
carborundum,  with  a  clay  bond  or  with  a  temporary  binder  such 
as  glue  or  dextrine.  In  the  latter  case  the  article  is  heated  in  an 
electric  furnace  until  the  carbide  recrystallizes,  yielding  a  strong, 
very  refractory  product,  known  as  "pure  crystolon. " 

Crystolon  is  a  better  conductor  of  heat  than  most  refractory 
materials  and  on  this  account  it  must  be  provided  with  a  heat  re- 
taining backing.  When  heated  it  becomes  a  fair  conductor  of  elec- 
tricity and  care  must  therefore  be  taken  not  to  place  it  where  it 
can  short-circuit  the  electric  current  passing  through  the  furnace. 

Silundum.2 — This  is  a  form  of  carbide  of  silicon  prepared  by 
heating  a  piece  of  carbon  in  the  vapor  of  silicon,  so  that  the  carbon 
is  converted  into  carbide.  This  is  mostly  used  as  a  resistor  (see 
pages  90  and  296)  but  it  can  also  be  employed  as  a  refractory 
material. 

Refractory  Materials  in  Electrical  Resistance  Furnaces,  F.  A.  J.  FitzGerald, 
Electrochemical  Industry,  vol.  ii,  (1904),  p.  439. 

Refractory  Materials  for  Furnace  Linings,  E.  K.  Scott,  Electrochemical 
Industry,  vol.  iii  (1905),  p.  140. 

1  FitzGerald,  Met.  and  Chern.  Eng.,  vol.  x  (1912),  p.  129. 

2  Silundum,  F.  Boiling,  Electrochem.  and  Met.  Ind.,  vol.  vii  (1909),  p.  24. 

5 


66 


THE  ELECTRIC  FURNACE 


Siloxicon.1 — This  is  made  in  the  same  manner  as  carborundum, 
but  less  carbon  is  used  in  the  charge,  with  the  result  that  the  silica 
is  not  completely  reduced,  and  the  resulting  substance  retains 
some  oxygen.  The  composition  is  not  constant,  as  a  series  of 
compounds  are  formed,  but  a  typical  formula  is  Si2C2O.  This 
forms  a  refractory  material  for  lining  furnaces,  and  may  be  made  to 
cohere  by  grinding  it  to  powder,  moistening  the  powder  with  water, 
pressing  it  into  a  mold,  and  strongly  firing  the  molded  material. 
The  firing  probably  oxidizes  the  siloxicon  grains  superficially,  forming 
silica  which  acts  as  a  bond.  Siloxicon  is  said  to  be  unaffected  by 
acid  or  basic  slags,  or  by  molten  iron,  but  although  this  may  be 
true  at  moderate  furnace  temperatures  it  can  scarcely  hold  at  the 
higher  temperatures  of  the  electric  furnace. 

The  silicon  carbides,  although  very  refractory,  are  slowly  oxidized 
at  high  temperatures  in  the  presence  of  air;  siloxicon  oxidizing 
when  heated  above  1,470°  C.,  or  2,674°  F.  Carborundum  was 
for  a  long  time  thought  to  be  unoxidizable,  but  it  has  been  found 
to  oxidize  slowly  at  high  temperatures. 

These  substances  are  far  less  refractory  than  carbon,  being 
dissociated  into  graphite  and  silicon  vapor  at  high  electric-furnace 
temperatures.2  They  can  be  used  in  some  forms  of  electric  furnace 

TABLE  V.— REFRACTORY  MATERIALS 


Material 

Melting  temperature 

Fire-clay  brick.     Kaolin  with  additional  silica  

Silica-brick.     Silica  with  binding  material  
Silica  (pure)                    

(  i,  600°  C. 
to 
1  1,730°  C. 
1,700°  C. 
i,7So0  C. 
1,820°  C. 

2,010°  C. 

2,050°  C. 
2,050°  C. 
2,180°  C. 
2,150°  C. 

2,200°  C. 
2,200°  C. 

3,600°  C. 

(  2,900°  F. 
to 
1  3,150°  F. 
3,ioo°  F. 
3,i8o°  F. 
3,3oo°  F. 
3,650°  F. 
3,7oo°  F. 
3,7oo°  F. 
3,950°  F. 
3,900°  F. 
4,000°  F. 
4,000°  F. 
6,500°  F. 

Bauxite  (impure  alumina)                         

Alumina  (pure) 

Lime  (pure)                     about 

Chrome-brick                                                    

Chromifo 

Magnesia-brick    

Magnesia  (pure)                            about 

Carborundum  SiC                                       decomposes 

Carbon  vaporizes  rapidly 

1  Oxidation  of  Siloxicon,  E.  G.  Acheson,  Electrochemical  Industry,  vol.  i,  p. 

373- 

Siloxicon  Brick,  Electrochemical  Industry,  vol.  i,  pp.  287  and  373;  vol.  ii,  p. 
442;  vol.  iii,  p.  445;  vol.  iv,  p.  40. 

2  S.  A.  Tucker  and  A.  Lampen,  Amer.  Chem.  Soc.,  vol.  xxviii,  p.  858,  find  the 
dissociation  temperature  of  carborundum  to  be  2,220°  C. 


CONSTRUCTION  AND  DESIGN  67 

as  a  layer  protecting  a  less  refractory  material  such  as  fire-clay  or 
magnesite-brick;  and  applied  as  a  paint,  mixed  with  silicate  of 
soda,  they  improve  very  materially  the  lasting  qualities  of  fire-clay 
bricks  in  ordinary  metallurgical  furnaces. 

THERMAL  CONDUCTIVITY  OF  FURNACE  MATERIALS 

In  addition  to  its  ability  to  resist  high  temperatures  and  cor- 
rosive slags,  the  power  of  a  furnace-lining  to  retain  the  heat  which 
is  produced  in  the  furnace  must  be  considered.  It  is  rare  that 
good  refractory  and  heat-retaining  qualities  are  combined  in  the 
same  material,  and  to  get  the  best  effect  it  is  usually  necessary 
to  adopt  a  stratified  construction,  placing  refractory  materials  on 
the  inside,  and  heat-retaining  materials  outside.  Generally  speak- 
ing, light  porous  substances  are  good  retainers  of  heat,  while  heavy 
compact  bodies  are  poor  heat-insulators.1 

Furnaces  have  in  general  been  designed  empirically  and  no 
calculation  has  been  made,  in  advance,  to  show  what  loss  of  heat 
would  take  place  by  conduction  through  the  walls,  and  what  effi- 
ciency might  be  expected.  During  the  last  few  years  the  methods 
of  calculation  have  been  improved  and  simplified2  and  some  addi- 
tional experimental  data  have  been  ascertained.  Before  very  long 
we  may  expect  to  have  a  complete  set  of  data  available,  and  that 
electric  furnaces,  and  to  a  less  extent  fuel-fired  furnaces,  will  be 
designed  in  a  scientific  manner  like  a  steam  engine  or  electric 
generator. 

Until  recently  very  little  information  was  available  with  regard 
to  the  rate  of  flow  of  heat  through  the  materials  of  furnace  walls, 
and  the  calculation  of  the  flow  of  heat  from  such  data  as  were 
available  was  not  well  understood. 

Heat  being  measured  in  gram-calories,  pound-calories,  British 
thermal  units,  etc.  (See  page  44)  the  flow  of  heat  has  usually  been 
stated  in  grain- colories  per  second  (or  in  similar  units)  corresponding 
for  example,  to  the  flow  of  water  in  gallons  per  minute. 

The  flow  of  heat  through  a  piece  of  brickwork  depends  on  the 
area  and  thickness  of  the  wall  through  which  the  heat  passes,  on  the 
difference  of  temperature  between  the  two  sides  of  the  wall  and  on 
the  thermal  conductivity  of  the  brickwork. 

1  See  also  a  paper  by  Hutton  and  Beard  on  Heat  Insulation.     Electrical 
Rev.,  N.  Y.,  July  22,  1905,  and  Eng.  Record,  Nov.  25,  1905. 

2  We  are  largely  indebted  for  this  improvement  to  a  number  of  valuable 
papers  by  Dr.  Carl  Hering.     A  list  of  these  papers  has  been  given  on  page  75. 


68 


THE  ELECTRIC  FURNACE 


If  AB  (Fig.  26.)  is  a  cube  of  unit  edge  (i  in.  or  i  cm.)  and  if  the  face 
A  is  kept  i°  hotter  than  the  face  B,  and  if  no  heat  enters  or  leaves 
the  cube  through  its  remaining  four  faces,  heat  will  flow  steadily 
from  A  to  B  and  the  amount  of  this  flow  is  numerically  equal  to  the 
thermal  conductivity  of  the  material.  If,  as  is  usual,  our  units  are 
centimeters,  grams,  seconds,  and  Centigrade  degrees,  the  conduc- 
tivity, k,  would  be  stated  as  so  many  gram-calories  per  second  for  a 
centimeter  cube,  for  one  Centigrade  degree  difference  of  temperature. 

It  has  been  found  that  the  flow  of  heat  through  a  body  is  propor- 
tional to  the  difference  of  temperature  causing  the  flow  (just  as  the 
flow  of  electricity  is  proportional  to  the  electromotive  force)  and  it 


\ 


FIG.  26. — Flow  of  heat 
through  unit  cube. 


k 1 — * 

FIG.  27. — Flow  of  heat  through 
section  of  wall. 


can  easily  be  shown  that  the  flow  will  be  proportional  to  the  area  of 
the  wall  and  inversely  proportional  to  the  thickness  of  the  wall 
through  which  it  flows,  just  as  in  the  flow  of  electricity. 

If  then  CD  (Fig.  27)  is  a  section  of  a  wall  through  which  the  heat 
flows  from  C  to  D  the  flow  of  heat  H  will  be  given  by  the  equation: 


Where  k  is  the  thermal  conductivity,  /  is  the  difference  of  tempera- 
ture between  C  and  D,  S  is  the  area  of  each  of  the  faces  C  and  D  and 
/  is  the  thickness  of  the  wall.  It  is  understood  in  this  case  that  C  and 
D  are  equal  and  parallel  and  that  the  flow  of  heat  is  perpendicular  to 
these  faces. 

In  calculating  the  loss  of  heat  from  a  furnace  we  are  met  by  the 
difficulty  that  the  outer  surfaces  of  the  furnace  walls  are  greater 
than  the  inner  surfaces  and  that  the  flow  of  heat  through  these  walls 


CONSTRUCTION  AND  DESIGN 


69 


is  not  parallel  but  radiating.  The  above  formula  cannot  be  applied 
directly  as  it  is  not  easy  to  see  what  is  the  effective  area  of  each  wall. 
This  difficulty  is  especially  serious  in  the  case  of  small  furnaces  with 
thick  walls. 

Fig.  28  represents  a  section  through  a  cubical  furnace.  The  inner 
surface  A  of  one  wall  is  at  a  temperature  (T-\-f)°  and  the  outer  surface 
B  at  a  temperature  7"°.  The  areas  are  s  and  S  respectively  and  the 
thickness  of  the  wall  is  /.  The  heat  flow  will  be  radiating,  as  shown 
by  the  figure,  and  we  wish  to  calculate  the  total  flow  of  heat  through 
the  wall  from  the  known  conductivity  of  a  unit  cube  of  the  material 
composing  it. 


FIG.  28. — Flow  of  heat  through 
side   of  hollow  cube. 


FIG.  29. — Flow  of  heat  through 
wall  of  hollow  sphere. 


It  can  be  shown  mathematically1  that  the  flow  of  heat  from  A 
to  B  will  be  given  correctly  by  the  formula: 


I 

In  other  words  the  effective  area  of  the  wall  AB  is  the  geometrical 
mean  of  its  inner  and  outer  surfaces.  The  same  applies  of  course  to 
the  remaining  walls  of  the  cube. 

The  same  formula  applies  correctly  in  the  case  of  a  spherical 
furnace,  Fig.  29,  having  an  inner  area  s  and  outer  area  S. 

The  calculation  of  the  flow  of  heat  from  most  electric  furnaces  can 
be  made  without  serious  error  by  assuming  the  walls  to  be  parts  of  a 
cubical  or  a  spherical  furnace. 

The  flow  of  heat  through  a  cylindrical  wall  (without  ends)  is 
given  by  the  more  complicated  formula: 

1  Carl  Hering,  Heat  Conductances  through  Walls  of  Furnaces.  Trans.  Am. 
Electrochem.  Soc.,  vol.  xiv  (1908),  p.  215. 


70  THE  ELECTRIC  FURNACE 


I  2.  3  03  (log  S—  log  s) 

This  formula  would  only  be  needed  in  the  case  of  a  very  long 
cylindrical  furnace  and  would  then  be  applied  to  the  cylindrical 
part  remote  from  the  ends.  Each  end  with  a  short  piece  of  the 
adjacent  wall  would  be  considered  as  the  half  of  a  sphere  or  cube. 

An  approximate  formula  for  the  flow  of  heat  through  a  cylindrical 
wall  is  given  by  the  equation:1 


iod 

Where  d  is  the  internal  diameter  of  the  cylinder,  and  /  its  thickness. 

In  the  above  formulae  it  has  been  assumed  that  the  conductivity, 
k,  is  a  constant,  although  it  is  well  known  that  the  thermal  conduc- 
tivity of  fire-bricks  and  similar  materials  increases  very  considerably 
with  a  rise  of  temperature.  The  correct  calculations  are  made  by 
using  the  above  formulae  with  the  "effective"  value  of  k  which  has 
been  shown  to  be  approximately  the  value  of  k  corresponding  to  the 
temperature  (T-}-t/2)0,  for  a  conductor  whose  ends  are  at  T°  and 
(r-H)°  respectively.2 

The  thermal  conductivity  of  furnace  materials  is  found  by  measur- 
ing the  flow  of  heat  through  a  slab  of  the  material  of  definite  dimen- 
sions having  the  opposite  faces  at  two  definite  temperatures, 
and  these  temperatures  are  mentioned  in  stating  the  result. 
Thus  the  conductivity  of  fire-brick  is  given  as  0.0014  (C.G.S. 
units)  between  the  temperatures  o°  and  500°  C.  and  0.0031  be- 
tween o°  and  1,300°  C.  These  values  are  not  the  conductivities 
at  500°  and  1,300°,  but  the  effective  conductivities  over  the  speci- 
fied ranges.  Very  little  is  known  with  regard  to  the  variation  of 
conductivity  with  temperature,  but  the  conductivity  rises  with  the 
temperature  and  in  some  cases  appears  to  be  nearly  proportional 
to  the  absolute  temperature. 

Making  the  assumption  that  the  thermal  conductivity  of  refrac- 
tory materials  is  a  linear  function  of  the  temperature  the  following 
rule  can  be  deduced: 

In  the  case  of  any  conductor,  whether  parallel  or  flaring,  the  effective 

1  Carl  Hering,  loc.  cit. 

2  Carl  Hering,  "Effects  of  the  Variations  of  Thermal  Resistivities  with  the 
Temperature."     Trans.  Am.  Electrochem.  Soc.,  xxi,  1912,  p.  511.     Dr.  Bering's 

T-\-T' 
formula  is  -    —  for  a  conductor  whose  ends  are  at  T°  and  (T')°. 


CONSTRUCTION  AND  DESIGN 


71 


conductivity  is  that  corresponding  to  the  mean  of  the  two  extreme 
temperatures.1 

In  the  case  of  a  parallel  conductor,  AB,  Fig.  30,  this  means  that 
the  flow  of  heat  from  A  to  B  is  given  by  the  equation: 

H  =  kt~ 
Where  k  is  the  conductivity  of  the  material  at  the  temperature 

(T+t/2)° 

The  experimental  data  give  the  effective  conductivities  between 
certain  temperatures;  thus  as  stated  above,  the  conductivity  of 
fire-bricks  between  o°  and  500°  is  0.0014  C.G.S.  units.  By  means 
of  the  rule  we  can  interpret  this  to  mean  that  the  conductivity  of 


(T*t)c 


\  __L\. 

\ 

1 
1 

IB 
i 

-A 

x 

FIG.  30. — Parallel  conductor  at      FIG.  31. — Flaring  conductor 


varying  temperatures. 


at  varying  temperatures. 


fire-brick  is  0.0014  at  250°  C.     Similarly  it  is  0.0031  at  650°  C. 
From  these  two  results  we  can  obtain  a  general  formula: 

kt=  0.000,34  +  0.000,004, 2  5/ 

If  only  one  determination  were  available  we  would  use  it  to  ob- 
tain the  constant  a  in  the  expression: 


assuming  that  the  conductivity  is  proportional  to    the   absolute 
temperature. 
In  the  case  of  a  flaring  conductor — such  as  CD,  Fig.  31,  which 

Carl  Hering,  loc.  cit. 


72  THE  ELECTRIC  FURNACE 

represents  one  wall  of  a  cubical  furnace,  it  was  shown  that  if  the 
conductivity  of  the  material  were  constant,  that  is  independent  of 
the  temperature,  the  flow  of  heat  from  C  to  D  would  be  given  by 
the  equation: 


where  /  is  the  difference  of  temperature  and  s  and  5  the  inner  and 
outer  surfaces.  Diagrammatically  this  may  be  taken  to  mean  that 
the  flow  of  heat  from  C  to  D  will  be  equal  to  the  flow  in  a  parallel 
conductor  AB  (Fig.  30)  having  the  same  length  /,  the  same  dif- 
ference of  temperatures  /,  and  a  uniform  cross-section  equal  to  \A  S. 
If  now  the  thermal  conductivity  varies  with  the  temperature  and 
may  be  assumed  to  be  a  linear  function  of  it,  the  flow  of  heat  from 
C  to  D  (Fig.  31)  will  be  equal  to  the  flow  from  A  to  B  (Fig.  30)  if 
the  conductivity  of  AB  is  throughout  equal  to  the  conductivity  of 
the  material  of  CD  for  the  temperature  (T-H/2)0. 

Thermal  conductivities  have  usually  been  stated  in  gram-calories 
per  second  for  a  centimeter  cube  with  one  degree  Centigrade  differ- 
ence of  temperature,  but  for  electrical  work  it  is  more  convenient 
to  express  the  flow  of  heat  in  watts,  one  watt  being  equal  to  0.239 
grm.-calories  per  second.  The  conductivity  would  then  be  ex- 
pressed as  so  many  watts  for  a  centimeter  cube  for  unit  difference  of 
temperature.1 

The  thermal  resistivity  of  a  substance  is  the  reciprocal  of  its  con- 
ductivity (as  in  electrical  measurements)  and  the  term  "  thermal 
ohm"  has  been  given  to  the  unit  of  thermal  resistivity  based  on  the 
watt.2 

The  thermal  ohm  is  the  thermal  resistance  of  a  body  through 
which  unit  difference  of  temperature  produces  a  flow  of  heat  equal 
to  one  watt. 

The  corresponding  unit  of  thermal  conductance3  is  the  thermal 
mho  and  is  the  conductance  of  a  body  through  which  unit  difference 
of  temperature  produces  a  flow  of  heat  equal  to  one  watt. 

1  Carl  Hering,  Met.  and  Chem.  Eng.,  viii,  1910,  p.  676. 

2  Carl  Hering,  "Thermal  Resistance  and  Conductance;  the  Thermal  ohm  and 
Thermal  Mho.,"  Met.  and  Chem.  Eng.,  ix,  1911,  p.  13. 

3  In  regard  to  the  use  of  these  terms  is  should  be  noted  that  as  in  electrical 
matters:     Resistivity  is  the  resistance  of  a  unit  cube  of  the  substance,  and 
replaces  the  older  term  "specific  resistance."     "  Conductivity"  is  the  "conduct- 
ance" of  a  unit  cube. 


CONSTRUCTION  AND  DESIGN  73 

Ohm's  law  holds  in  regard  to  the  flow  of  heat  just  as  in  electrical 
matters. 

If  a  conductor  of  heat  has  a  thermal  resistance  R  (in  thermal 
ohms)  and  a  difference  of  temperature  /  (in  Centigrade  degrees)  be- 
tween the  ends,  the  flow  of  heat  H,  in  watts,  is  given  by  the  equation: 

-» 

Table  VI  is  a  collection  of  experimental  data  on  the  thermal  con- 
ductivity and  resistivity  of  furnace  materials. 

The  values  have  been  given  in  conductivities  (C.G.S.  units)  to 
allow  of  easy  comparison  with  other  published  data,  and  in  thermal 
ohms  as  these  are  the  most  useful  in  electric-furnace  calculations. 
The  resistivities  have  been  expressed  in  terms  of  the  inch  cube,  as 
well  as  the  centimeter  cube,  as  inches  are  still  in  very  general  use. 

It  will  be  understood  that  a  material  having  a  high  conductivity 
for  heat,  as  shown  in  the  table,  would,  if  used  in  the  construction  of 
a  furnace  wall,  allow  a  considerable  amount  of  heat  to  escape  and 
be  wasted.  Light  powders  like  infusorial  earth  are  good  for  retain- 
ing the  heat  in  a  furnace,  but  they  do  not  keep  their  heat-insulating 
qualities  at  high  temperatures  and  should  only  be  used  as  an  outer 
jacket  to  the  furnace.  Undoubtedly  much  could  be  gained  in  ordi- 
nary furnaces  by  a  more  careful  attention  to  the  heat-conducting 
qualities  of  the  materials  of  which  the  walls  are  composed,  and  in 
electric  furnaces,  where  the  cost  of  the  heat  is  usually  considerably 
greater,  it  is  even  more  important  to  guard  as  far  as  possible  against 
loss.  On  the  other  hand,  cases  are  common  in  large  fuel-fired  fur- 
naces, and  occur  even  in  electric  heating,  where  the  importance  of 
preserving  some  portion  of  the  furnace,  that  is  exposed  to  corrosive 
slags  or  very  high  temperatures,  is  greater  than  the  need  to  save  the 
heat,  and  in  such  cases,  air-cooling,  and  even  water-cooling  of  the 
furnace  walls  may  be  adopted.  It  should  be  remembered  that 
the  rate  of  loss  of  heat  from  a  furnace  will  be  proportional  to  the 
area  of  its  walls,  that  is,  to  the  square  of  the  linear  dimensions. 
The  ratio  of  heat-loss  per  unit  volume  will,  therefore,  be  inversely 
proportional  to  the  dimensions  of  the  furnace,  or  a  furnace  that 
is  twice  as  large  as  another  (in  linear  dimensions)  will  only  have 
half  as  large  a  heat-loss,  for  a  given  volume  of  the  interior  of  the 
furnace.  This  supposes  the  furnace  walls  to  be  of  equal  thickness 
in  the  two  furnaces,  but  in  small  experimental  furnaces  the  walls 
are  usually  thinner  than  they  are  in  full-sized  furnaces,  and  under 


74 


ELECTRIC  FURNACE 


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CONSTRUCTION  AND  DESIGN  75 


NOTES  ON  TABLE 

Column  I  shows  the  flow  of  heat  in  gram-calories  per  second  through  a  cube 
of  i-cm.  edge  due  to  a  difference  of  temperature  of  i°  C. 

Column  II  shows  the  difference  of  temperature  in  Centigrade  degrees  that 
would  be  required  to  produce  a  heat-flow  of  i  watt  through  a  cube  of  i-cm.  edge. 

Column  III  is  the  same  as  column  II  but  for  a  cube  of  i-in.  edge. 

The  figures  in  column  II  are  obtained  by  dividing  0.239  by  the  figures  in 
column  I. 

The  figures  in  column  III  are  obtained  by  dividing  the  figures  in  column 
II  by  2.54. 

REFERENCES 

H.      Det.  by  Carl  Hering,  Met.  and  Chem.  Eng.,  vol.  ix  (1911),  p.  653. 
N.H.  Det.  by  Nusselt,  quoted  by  Hering,  he.  cit. 
O.H.    Det.  by  Ordway,  quoted  by  Hering,  loc.  cit. 
P.H.   Det.  by  Peclet,  quoted  by  Hering,  loc.  cit. 

R.  Richards'  Metallurgical  Calculations,  vol.  i,  pp.  183-4. 

W.  Det.  by  Wologdine,  Met.  and  Chem.  Eng.,  vol.  vii  (1909),  p.  383. 

PAPERS  BY  CARL  HERING  ON  THE  THEORY  OF  THERMAL  CONDUCTIVITY 

"Heat  Conductances  through  Walls  of  Furnaces."  Trans.  Am.  Electrochem. 
Soc.,  vol.  xiv  (1908),  p.  215. 

"Heat  Conductance  and  Resistance  of  Composite  Bodies."  Electrochem. 
and  Met.  Ind.,  vol.  vii  (1909),  p.  n. 

"Heat  Conductivities  in  the  Calculation  of  Furnaces."  Electrochem.  and 
Met.  Ind.,  vol.  vii  (1909),  p.  72. 

"Thermal  Resistance  and  Conductance;  the  Thermal  Ohm  and  Thermal 
Mho."  Met.  and  Chem.  Eng.,  vol.  ix  (1911),  p.  13. 

"The  Flow  of  Heat  through  Bodies."  Met.  and  Chem.  Eng.,  vol.  ix  (1911), 
p.  652. 

"Flow  of  Heat  through  Contact  Surfaces."  Met.  and  Chem.  Eng.,  vol.  x 
(1912)  p.  40. 

"The  Thermal  Insulation  of  Furnace  Walls."  Met.  and  Chem.  Eng.,  vol.  x 
(1912),  p.  97 

"The  Flare  of  Furnace  Walls."    Met.  and  Chem.  Eng.,  vol.  x  (1912),  p.  159. 

"Effects  of  the  Variations  of  Thermal  Resistivities  with  the  Temperature." 
Trans.  Am.  Electrochem.  Soc.,  vol.  xxi  (1912),  p.  511. 


76  THE  ELECTRIC  FURNACE 

these  conditions  the  small  furnace  fares  even  worse  in  proportion. 
In  the  extreme  case  of  a  small  furnace  constructed  as  an  exact 
model  on  a  scale  of  one  inch  to  the  foot  of  a  large  furnace,  the  heat- 
loss,  for  each  cubic  inch  of  the  model,  would  be  144  times  as  great 
as  from  the  large  furnace,  provided,  of  course,  that  both  attained 
the  same  temperature.  In  other  words  if  the  furnaces  were  merely 
being  kept  hot,  no  work  being  done  in  them,  the  small  furnace  would 
need  144  times  as  much  heat  per  cubic  inch  as  the  large  furnace  in 
order  to  keep  it  heated  to  the  same  temperature. 

Some  very  careful  work  on  the  thermal  conductivity  of  refrac- 
tory materials  has  been  done  by  S.  Wologdine  (reference  in  notes 
to  table)  and  he  shows  that  the  conductivity  increases  with  the 
temperature  at  which  the  brick  has  been  burnt. 

The  determination  of  the  thermal  resistivity  of  a  brick  is  usually 
made  by  heating  one  side  of  the  brick  by  means  of  a  flame  and  allow- 
ing the  heat  that  flows  through  the  brick  to  pass  into  a  calorimeter 
which  is  applied  to  the  other  side  of  the  brick.  Thermo-couple 
pyrometers  are  used  to  determine  the  temperature  of  both  sides  of 
the  brick,  the  flow  of  heat  is  measured  by  the  calorimeter,  and  the 
thickness  and  effective  cross-section  of  the  part  of  the  brick  through 
which  heat  enters  the  calorimeter  is  known.  The  thermal  con- 
ductivity can  be  calculated  from  these  data.  In  such  tests  it  is 
necessary  to  allow  the  heat  to  flow  for  some  hours,  until  the  tem- 
perature at  each  point  in  the  brick  becomes  perfectly  steady, 
before  making  an  observation. 

Another  method  is  to  construct  a  furnace  of  the  material  to  be 
tested  and  to  generate  heat,  electrically,  inside  the  furnace.  When 
the  temperature  in  the  furnace  has  become  steady,  the  flow  of 
heat  through  the  walls  is  equal  to  the  power  supplied  to  the 
furnace.  The  temperature  of  the  inner  and  outer  surfaces  of  the 
furnace  must  then  be  measured,  and  from  these  observations  and 
the  dimensions  of  the  furnace,  the  thermal  conductivity  of  the 
brickwork  can  be  calculated. 

The  loss  of  heat  from  an  electric  furnace  depends  mainly  on  the 
thermal  conductance  of  the  furnace  walls,  but  it  is  modified  by 
what  is  known  as  the  " contact  resistance"  between  the  outside 
of  the  furnace  walls  and  the  surrounding  air,  and  between  the  in- 
side of  the  walls  and  the  hot  gases  or  other  materials  within  the 
furnace.  Thus  if  the  gases  within  an  electric  furnace  were  at  1,500° 
C.,  and  the  outer  air  was  at  20°  C.,  the  inside  of  the  furnace  walls 
would  not  be  at  1,500°  C.,  but  at  some  lower  temperature,  say 


CONSTRUCTION  AND  DESIGN  77 

1,450°  C.,  and  the  outside  of  the  furnace  walls  would  not  be  at  20° 
C.,  but  at  some  higher  temperature,  say  100°  C.  These  differ- 
ences of  temperature  between  the  walls  and  the  adjacent  gases 
are  due  to  the  thermal  resistance  at  the  contact,  and  the  flow  of 
heat  through  the  wall  is  less  than  it  would  be  if  the  inside  of  the 
wall  were  at  1,500°  C.  and  the  outside  at  20°  C. 

If  we  can  measure  the  temperatures  of  the  inside  and  the  out- 
side of  the  walls  we  can  calculate  the  flow  of  heat  through  the  walls 
and  this  gives  us  the  whole  loss  of  heat.  If,  however,  we  only  know 
the  temperature  of  the  gases  in  the  furnace  and  of  the  air  outside 
the  furnace,  we  shall  need  to  know  the  thermal  resistance  of  the 
contacts,  before  we  can  calculate  the  loss  of  heat. 

The  conductivity  and  resistivity  determining  the  flow  of  heat 
through  contact  surfaces  are  expressed  in  a  manner  similar  to  the 
thermal  conductivity  and  resistivity  of  conducting  bodies,  except 
that  contact  conductivity  and  resistivity  are  expressed  in  terms 
of  the  unit  square  of  contact  surface  instead  of  the  unit  cube  of 
conducting  material1 

RADIATION  AND  CONVECTION  OF  HEAT 

The  loss  of  heat  from  the  outside  of  a  furnace  is  due  partly  to 
conduction — that  is,  the  transfer  of  heat  (by  molecular  motion) 
to  immediately  adjacent  bodies,  such  as  the  supporting  masonry 
and  the  air  surrounding  the  furnace;  and  partly  to  radiation — that 
is,  the  transfer  of  heat  from  the  furnace  by  vibrations  in  the  lumin- 
iferous  ether.  Radiant  heat  passes  with  little  loss  through  air  and 
other  gases  but  is  partly  reflected  and  largely  absorbed  when  it 
meets  solid  or  liquid  substances. 

The  loss  of  heat  by  conduction  through  solid  substances  has 
already  been  considered.  The  loss  of  heat  by  conduction  to  the 
surrounding  air  is  very  greatly  increased  by  "convection" — that  is, 
the  movement  of  the  heated  air  away  from  the  furnace,  carrying 
with  it  the  heat  which  it  received  by  conduction  from  the  furnace 
walls. 

The  loss  of  heat  by  radiation  is  very  small  from  polished 
metals  and  varies  considerably  with  different  metals  and  their 
degree  of  polish,  but  in  the  case  of  rough  substances,  such  as  the 
black  iron-work,  or  brick  work,  of  which  furnaces  are  built,  the 
rate  of  loss  is  almost  independent  of  the  nature  of  the  radiating 

1  Numerical  values  are  given  in  Table  VII. 


- 

78  THE  ELECTRIC  FURNACE 

surface.  The  rate  of  loss  from  such  a  body  at  100°  C.,  placed  in 
a  vacuum,  so  that  there  is  no  loss  by  convection,  and  surrounded 
by  bodies  at  o°  C.  is  about  0.015  grnvcalories  per  second  for  each 
square  centimeter  of  radiating  surface.  The  rate  of  loss  is  propor- 
tional to  the  difference  between  the  fourth  powers  of  the  absolute 
temperature  (273+0°)  of  the  hot  body  and  that  of  surrounding 
bodies.  Hence  it  is  possible  to  calculate  the  loss  by  radiation  from 
a  hot  body  if  its  temperature  and  that  of  surrounding  reflecting 
or  radiating  bodies  are  known. 

The  loss  of  heat  by  conduction  and  convection  depends  to  some 
extent  on  the  nature  of  the  heated  body,  but  to  a  greater  extent  on 
the  fluid  that  serves  to  carry  the  heat  away.  The  rate  of  loss  can 
be  determined  if  we  know  the  contact  resistivity,  that  is  the 
difference  of  temperature,  in  degrees  Centigrade,  between  the  hot 
body  and  the  surrounding  fluid,  that  would  cause  a  flow  of  i  grm.- 
calorie  per  second  from  each  square  centimeter  of  the  hot  surface. 
In  the  case  of  the  conduction  and  convection  of  heat  from  metal  to 

air  the  resisitivity  is  given  by  the  expression1  R=          .-'  in  which  v 

2  +  Vv 

denotes  the  velocity  of  the  air  in  centimeters  per  second. 

If,  as  in  the  previous  case,  the  hot  body  were  at  100°  C.  and  its 
surroundings  at  o°  C.,  and  if  the  velocity  of  the  air  were  10  cm.  per 
second  (caused  by  the  heat  from  the  hot  body),  the  contact 

resistivity,  R,  would  be        '    .  —  =  7,000  and  the  rate  of  loss  of  heat 

2+Vio 
\ 

from  each  square  centimeter  of  the  heated  surface  would  be 

7000 

=0.014  grm.-calorie  per  second.  The  loss  by  radiation  was  0.015 
grm.-calorie  per  second,  making  a  total  loss  of  0.029  cal.  per  second 
per  square  centimeter. 

If  now  the  hot  body  were  at  200°  C.,  with  surroundings  at  o°  C. 
as  before,  the  rate  of  loss  of  heat  by  radiation  would  be: 

O.OI5X/       \4     /       >.4  =  o.o48  grm.-calorie  per  second. 

The  loss  of  heat  by  conduction  and  convection,  assuming  the 
velocity  of  the  air  to  be  15  cm.  per  second,  would  be: 


ioo°-o° 


/       o       ox       36,000        200 
(200°  -o0)-J—   ^-=  = 


=  0.033 


6,100 
1  Richards  "Metallurgical  Calculation,"  vol  i,  p.  179. 


CONSTRUCTION  AND  DESIGN 


79 


The  whole  loss  of  heat  will  in  each  case  be  the  sum  of  the  losses  by 
radiation  and  by  conduction  and  convection,  namely  0.029  at  100°  C. 
and  0.081  at  200°  C. 

In  the  case  of  electric  furnaces  where  the  loss  of  heat  is  partly  by 
radiation  and  partly  by  air  convection,  and  when  the  motion  of  the 
air  is  only  that  caused  by  the  heat  of  the  furnace,  it  is  possible  to 
obtain  a  figure  for  what  may  be  termed  the  effective  contact  re- 
sistivity, governing  the  whole  transfer  of  heat  from  a  hot  body  to 
the  air  and  surrounding  bodies. 

The  following  table  shows  the  rate  of  loss  of  heat  from  the  outside 
of  furnaces  surrounded  by  air  at  20°  C.  The  loss  is  given  in  watts 
per  square  inch  and  kilowatts  per  square  foot  of  the  surface  when 
this  is  at  various  temperatures.  The  contact  conductivity  would  be 
obtained  by  dividing  this  flow  of  heat  by  the  difference  of  tempera- 
ture causing  it,  and  the  reciprocal  of  this  is  the  contact  resistivity 
which  is  given  in  the  table.  The  contact  resistivity  is  the  difference 
of  temperature,  between  the  furnace  and  the  surrounding  air,  divided 
by  the  flow  of  heat  from  unit  area  of  the  surface. 

TABLE  VII.— FLOW  OP  HEAT  AND  CONTACT  RESISTIVITY* 
(From  heated  brick  and  iron  to  air  at  20°  C.) 


Flow  of  heat 

Contact  resistivity 

K.  W. 

per 
square 
foot 

Watts 
per 
square 
inch 

C.  G.  S. 

units 

Thermal  ohms 

per  square 
centimeter 

per  square 
inch 

Brick  at    90°  C  

0.09 
0.13 
o.  20 
0.30 
0-39 

0.08 
o.n 
0.18 
0.27 
0-34 

0.63 
0.90 
i-4 

2.1 

2.7 

3,020 
2,700 
2,540 
2,190 

2,000 

722 

645 
607 

523 
477 

112 
100 

94 
81 

74 

no0  C  

150°  C  
190°  C  

220°  C  

Iron  at      90°  C 

0.56 
0.76 

1.25 
1.9 
2.4 

3,4oo 
3,180 
2,810 
2,460 
2,300 

812 
760 
671 
587 
548 

126 
118 
104 
9i 
85 

110°  C  

150°  C  
190°  C  

220°  C  

The  loss  of  heat  from  bricks  at  100°  C.  surrounded  by  air  at  o°  C. 
would  be: 


0.035  grm.-calorie  per  second  for  i  sq.  cm. 


1  Compiled  from  data  by  F.  T.  Snyder,  Am.  Electrochem.  Soc.,  xviii,  1910,  p. 
239.  The  flow  of  heat  was  calculated  from  these  data  by  Carl  Hering,  Met.  and 
Chem,  Eng.,  x,  1912,  p.  41. 


80  THE  ELECTRIC  FURNACE 

and  if  the  bricks  were  at  200°  C.,  the  loss  would  be: 

200°  — o° 

—  =  0.094  calorie  per  second  for  i  sq.  cm. 
2130 

These  figures  do  not  agree  exactly,  but  are  of  the  same  order  as 
those  previously  obtained,  namely  0.029  and  0.081, respectively. 

In  calculating  the  loss  of  heat  from  a  furnace  we  determine  the 
thermal  resistance  of  the  furnace  wall,  and  add  the  contact  resistance 
from  the  walls  to  the  surrounding  air.  Dividing  the  differ eirce  of 
temperature  between  the  interior  of  the  furnace  and  the  external 
air,  by  the  total  resistance  in  thermal  ohms,  will  give  the  loss  of 
heat  in  watts.  In  doing  so  we  are  neglecting  the  contact  resistance 
within  the  furnace,  but  this  is  probably  very  low  and  its  omission 
does  not  cause  any  serious  error. 

Instead  of  stating  the  contact  resistivity  from  a  furnace  wall  to 
the  surrounding  air  it  is  sometimes  convenient  to  state  the  loss  of  heat 
in  watts  from  each  square  inch  of  the  surface.  The  loss  of  heat,  in 
watts,  is  equal  to  the  difference  of  temperature  divided  by  the  re- 
sistivity in  thermal  ohms. 

If  the  surroundings  are  at  20°  C.  the  rate  of  loss  of  heat  will  be 
about  i  watt  per  square  inch  at  120°  C.,  and  3  watts  per  square  inch 
or  0.4  kw.  per  square  foot  at  230°  C.  At  a  dull  red  heat  the  loss  will 
be  about  7  to  u  kw.  per  square  foot  and  at  a  bright  red  heat  12  to  15 
kw.  per  square  foot.1 

F.  A.  J.  FitzGerald2  built  a  furnace  4j-in.  cube  inside  and  i\ 
in.  thick  to  determine  the  loss  of  heat  through  furnace  walls.  The 
furnace,  Fig.  32,  was  heated  by  a  nichrome  resistor,  and  the  internal 
temperature  corresponding  to  a  definite  rate  of  heat  production  was 
measured.  Tests  were  made  with  the  furnace  composed  of  seve- 
ral kinds  of  refractory  materials.  In  one  test,  with  the  furnace 
built  of  firebricks,  the  internal  temperature  was  825°  C.  It  will 
be  of  interest  to  calculate  the  probable  loss  of  heat  from  this 
furnace  and  to  compare  the  result  with  the  observed  rate  of  heat 
production. 

This  furnace  was  not  quite  regular  in  outline  but  may  be  assumed 
to  be  a  9  i/2-in.  cube  externally,  the  surrounding  air  being 
assumed  to  be  at  25°  C. 

1  Carl  Hering,  Met.  and  Chem.  Eng.,  Jan.,  1912,  vol.  x,  p.  41.     F.  T.  Snyder, 
Am.  Elctrochem.  Soc.,  vol.  xviii,  p.  239.     Richards,  Metal  Calculations,  vol. 
i,  Chap.  viii. 

2  FitzGerald,  "Experiments  in  Heat  Insulation,"  Am.  Electrochem.  Soc.,  1912, 
vol.  xxi,  p.  535. 


CONSTRUCTION  AND  DESIGN 


81 


The  furnace  is  composed  of  six  faces  each  having  an  inner  surface 
of  4.5  in.  square  or  20.2  sq.  in.,  and  an  outer  surface  of  9.5  in.  square 
or  90  sq.  in.  The  effective  area  is: 


sq.  in. 
As  the  heater  in  this  furnace  is  not  in  contact  with  the  walls  there 


FIG.  32. — Furnace  for  testing  heat-losses. 

will  be  a  contact  resistance  between  the  heated  gases  within  the 
furnace  and  the  inside  of  the  furnace  walls.  We  have  no  reliable 
figure  for  the  contact  resistivity  at  this  temperature,  but  assuming 
it  to  be  30  thermal  ohms  per  square  inch  there  will  be  a  resistance 


82  THE  ELECTRIC  FURNACE 

for  one  face  of  -  =i-49  ohms.  The  resistivity  of  the  brick  wall 
will  be  about  40  thermal  ohms  for  i  in.  cube  giving  a  resistance  for 
one  face  of  40  X  —-7  =  2.35  ohms.  The  external  contact  resistivity 

may  be  about  85  ohms  per  square  inch  giving  a  resistance  of  —  = 

0.94  ohm. 

The    total    resistance  per  face=  1.49+2.  35+0.94  =  4.78   ohms. 

This  gives  a  heat  loss  for  the  whole  furnace  of  --  5—  =  1,000    watts. 

4.75 

This  agrees  sufficiently  well  with  the  actual  loss,  which  was  equal  to 
the  rate  of  heat  production  —  900  watts. 
The  temperature  of  the  outside  face  of  the  furnace  will  be 


That  of  the  inside  face  will  be 


As  a  further  example  we  may  calculate  the  loss  of  heat  from  the 
Heroult  steel  furnace,  Fig.  93,  referred  to  on  page  214.  In  this 
case  we  may  assume  the  whole  interior  of  the  walls  and  roof  to  be  at 
1,500°  C.,  the  gases  above  the  molten  charge  being  at  a  higher  tem- 
perature. In  this  furnace  the  roof  is  thinner  than  the  walls  and 
bottom  of  the  furnace  and  of  a  different  material,  and  in  consequence 
the  calculation  will  be  somewhat  more  complicated. 

The  roof  is  9  in.  thick  of  silica  brick  having  a  resistivity  (between 
1,000°  and  150°  C.)  of  30  thermal  ohms  for  i-in.  cube.  Its  inner 
surface  is  84  in.  X  50  in.  =4,200  sq.  in.  We  may  assume  that  the 
flare  corresponds  to  an  angle  of  45°  making  an  external  surface  of 
102  in.X68  in.  =  6,900  sq.  in.,  or  a  mean  area  of  \/4>  200X6,  900  = 


9 


\/29,ooo,ooo  =  5,400  sq.  in.    The  resistance  is  30  X—   —  =  0.05  tiler- 
s'00 

mal  ohm.     The  contact  resistance,  assuming  an  external  tempera- 
ture of  400°  C.,  will  be  about  40  thermal  ohms  per  square  inch  or 

=  0.006  ohm  for  the  whole  surface.     The  total  resistance  of 


6,900 

the    roof    will    therefore  be  0.056   ohms  and   the  flow  of   heat 

o  o 

-&-  =  26  kw.     It  will  be  noticed  that  we  have  not  taken  any 


0.056 


CONSTRUCTION  AND  DESIGN  83 

account  of  the  loss  of  heat  by  conduction  along  the  electrodes,  but 
on  the  other  band  have  included  their  area  as  part  of  the  roof. 
The  conduction  of  heat  along  the  electrodes  should  be  calculated 
separately,  see  page  99. 

The  walls  and  bottom  are  made  of  dolomite  bricks  and  crushed 
dolomite.  We  have  no  figure  for  the  resistivity  of  this,  but  that  of 
magnesite  bricks  is  13  thermal  ohms  (from  200°  —900°)  and  we  may 
assume  it  to  be  a  little  higher,  say  20;  ordinary  fire-bricks  being  22. 
The  bottom  may  be  taken  as  80  in.X45  m->  the  sides  80  in.X24  in. 
and  the  ends  45  in.X24  in.  inside.  The  thickness  may  be  taken 
as  12  in.  and  the  external  dimensions  each  24  in.  more  than  the  in- 
ternal. The  effective  area  of  the  whole  of  these  will  be  15,200  sq.  in. 

,  .  .          .  ,          12X20        240    . 

and  the  resistance  -       —  =  —      —•'=0.016  thermal  ohm. 
15,200     15,200 

The  contact  resistivity  may  be  taken  as  70  and  the  whole  contact 
resistance  will  be: 

70  ohms  per  square  inch 
24,000  sq.  in.  external  surface. 

=  0.003  onm>  giving  a  total  resistance  for  the  sides  and  bottom  of 

o  o 

0.019  thermal  ohm  and  a  flow  of  heat  equal  to  -^  —  —  -  —  =  77  kw. 

0.019 

The  total  loss  of  heat  by  conduction  and  radiation  from  the  furnace 
will  therefore  be77+26  =  io3  kw. 

The  power  used  was  350  kw.,  so  that  the  loss  by  conduction  would 
be  about  30  per  cent,  of  the  whole,  allowing  a  possible  efficiency  of 
70  per  cent. 

The  temperature  of  the  outside  of  the  furnace  roof  will  be 


i880  C. 
and  the  outside  of  the  walls 

2620  C. 


In  view  of  the  uncertainty  in  regard  to  the  constants  and  the  other 
assumptions  made  in  these  calculations,  we  cannot  count  on  great 
accuracy  in  the  results;  but  they  give  some  information  and  we  may 
hope  that  before  long  we  may  have  more  accurate  data  and  can  make 
calculations  sufficiently  exact  for  our  requirements. 

Furnace  with  External  Gas-heating.—  The  loss  of  heat  from  elec- 
tric furnaces  can  sometimes  be  made  less  by  heating  the  outside 


84 


THE  ELECTRIC  FURNACE 


of  the  walls  by  means  of  fuel.     This  would  only  be  practicable 
when  very  cheap  fuel-heat  was  available. 

Fig.  33 l  represents  a  cross-section  of  a  carborundum  furnace 
having  a  number  of  holes  in  the  lateral  walls.  The  gases  resulting 
from  the  reaction  escape  through  these  holes  and  burn,  heating 
the  outside  of  the  walls.  Screens,  E,  are  placed  outside  the  walls 
for  the  purpose  of  retaining  the  heat  produced  by  the  burning  gas. 
Holes,  F,  in  the  bottom  of  the  screens  admit  sufficient  air  for  the 
combustion  of  the  gas.  The  efficiency  of  external  heating  can 
only  be  small,  and  the  method  would  often  be  very  inconvenient.  A 
similar  principle  is  employed  in  the  Harker  tube  furnace,  Fig.  70. 


FIG.  33. — Furnace  with  external  gas-heating. 

Furnace  Walls  without  Refractory  Materials. — The  properties 
of  a  number  of  refractory  materials  have  been  considered,  but  it 
not  infrequently  happens,  in  electric-furnace  construction,  that 
the  heat  can  be  developed  in  the  midst  of  a  large  mass  of  the  ma- 
terial to  be  heated;  and  although  a  very  high  temperature  may  be 
reached  internally,  the  exterior  never  becomes  strongly  heated, 
and  mere  retaining  walls,  which  need  not  be  extremely  refractory, 
can  be  used.  The  best  known  example  of  this  is  the  Acheson 
furnace  for  the  production  of  carborundum,  Fig.  8.  The  Willson 
carbide  furnace,  Fig.  7,  also  depends  for  its  preservation  upon  the 
unacted  on,  and  relatively  cool  portions  of  the  charge,  as  the  walls 
of  the  crucible  are  only  made  of  iron. 

The  same  principle  can  be  applied  in  the  case  of  -continuous 
electric  smelting  furnaces,  by  constructing  the  furnace  in  such  a 
way  that  the  heat  is  developed  within  the  mass  of  ore  descending 
in  the  shaft  of  the  furnace,  and  by  regulating  the  current  so  that 

1  W.  A.  Smith,  U.  S.  patent  935,  937.  Electrochem.  and  Met.  Ind.,  vii,  1909, 
p.  494. 


CONSTRUCTION  AND  DESIGN  85 

a  portion  of  the  ore  will  remain  unfused  around  the  sides  of  the 
furnace.  When  this  can  be  done,  no  trouble  will  be  experienced 
in  maintaining  the  walls  for  an  indefinite  period,  even  when  cor- 
rosive slags  are  produced;  but  this  method  does  not  lend  itself 
readily  to  processes  in  which  the  charge  must  be  heated  considerably 
above  its  melting-point,  as  the  hot  central  portion,  being  liquid, 
will  mix  with  the  cooler  parts  round  the  sides,  and  will  eventually 
fuse  the  whole  of  the  protecting  layer  of  ore. 

The  device  of  restricting  the  zone  of  highest  temperature  to  the 
middle  of  a  furnace  depends  upon  a  constant  abstraction  of  heat 
around  the  sides.  This  is  usually  the  result  of  the  air-cooling  of 
the  outer  walls,  but  it  would  be  more  ideal  if  the  cooling  of  the  walls 
could  be  effected  by  a  continual  supply  of  fresh  ore,  so  that  the 
heat  would  not  really  be  wasted,  but  would  be  used  in  heating 
the  fresh  ore.  In  some  cases,  however,  it  is  even  necessary  to 
water-cool  parts  of  furnaces  in  order  to  preserve  the  walls. 

As  an  example  of  this  may  be  mentioned  the  De  Laval  smelting 
furnace,  Fig.  20.  This  has  two  troughs,  B  and  C,  which  contain 
molten  metal  and  serve  as  electrodes,  and  these  are  separated  by  a 
partition.  The  partition  being  entirely  within  the  furnace  will 
experience  very  little  air-cooling,  and  the  arrangement  of  the 
electrodes  tends  to  make  the  current  flow  most  strongly  against 
it  in  passing  through  the  slag.  The  partition  will  consequently 
become  very  hot  at  this  point,  and  would  certainly  dissolve  away, 
if  it  were  not  for  the  cooling  effect  of  the  water-jacket  /  placed 
within  it. 

As  further  examples  of  water-cooling,  may  be  mentioned  the 
water-cooled  electrodes  in  Heroult's  electric  steel-furnace.  The 
electrode  is  cooled,  by  a  water-jacket,  at  the  point  where  it  passes 
through  the  furnace  roof,  and  the  part  exposed  to  the  air  is  therefore 
kept  below  a  red  heat,  and  does  not  oxidize  as  it  otherwise  would. 
With  this  arrangement,  a  closer  joint  is  maintained  around  the 
electrode,  the  roof  is  protected  from  cutting  by  the  flame  issuing 
from  the  furnace,  and  less  loss  of  heat  occurs. 

Another  use  of  water-cooling  is  in  electrolytic  furnaces,  when  the 
molten  electrolyte  is  contained  in  an  iron  vessel,  which  is  required 
to  be  gas-tight.  Since  both  the  electrodes  pass  through  the  walls 
of  the  vessel,  or  the  vessel  itself  may  be  one  electrode,  it  is  necessary 
to  introduce  an  insulating  joint  at  some  point,  and  this  joint  must 
be  unaffected  by  heat,  by  the  electrolyte,  or  by  the  gases  given  off 
in  the  operation.  A  satisfactory  method  of  effecting  this  is  to 


86  THE  ELECTRIC  FURNACE 

make  the  vessel  in  two  parts,  one  of  which  may  be  the  lid,  and  to 
maintain,  by  water-cooling,  a  layer  of  solidified  electrolyte  between 
the  two  parts,  which  are  slightly  separated.  This  method  is 
employed  in  Borchers'  appliances  for  the  electrolysis  of  fused  zinc 
chloride,  and  for  the  electrolysis  of  fused  salts  of  lead.1 

Electrode  holders  are  sometimes  water-cooled,  to  prevent  them 
from  becoming  unduly  heated,  and  occasionally  even  the  electrodes 
themselves  are  water-cooled,  as  the  metal  tube  electrode  in  Siemens' 
arc  furnace,  Fig.  3,  or  the  water-cooled  electrodes  in  nitric- acid 
furnaces,  Figs.  139  and  140,  or  in  Gin's  steel  furnace,  Fig.  108. 

RESISTORS  2 

The  materials  employed  as  resistors  determine  very  largely  the 
voltage  of  electric  furnaces,  and  have  been  referred  to  under  that 
heading;  but  it  will  be  convenient  to  consider  them  particularly 
at  this  point. 

Three  cases  present  themselves:  (i)  Arc  furnaces — in  which  the 
resistor  consists  of  the  intensely  heated  gases  and  vapors  in  the  arc. 
(2)  Furnaces  in  which  the  current  passes  through,  and  directly 
heats,  the  charge  itself.  (3)  Furnaces  having  a  special  resisting 
conductor,  in  which  the  heat  is  developed. 

(1)  The  arc  furnaces  need  not  be  specially  considered,  as  any 
gases  or  vapors  that  are  ordinarily  present  in  electric  furnaces, 
will  serve  to  carry  the  current. 

(2)  Furnaces  in  which  the  Current  Passes  through  the  Charge. — 
More  furnaces  belong  to  this  class  than  to  class  (3);  and  it  will 
obviously  be  more  satisfactory,  when  possible,  to  pass  the  current 
through  the  material  of  the  charge,  instead  of  providing  a  special 
resistor  for  this  purpose.     The  electrical  conductivity  of  the  charge 
will  usually  determine  whether  or  not  it  can  be  used  as  a  resistor.     Of 
the  ordinary  materials  found  in  nature,  only  the  metals  and  carbon 
are  sufficiently  conductive  to  carry  large  electric  currents;  but, 
when  heated  to  their  melting  temperature,  most  of  the  rock-forming 
minerals  will  carry  an  electric  current;  and  when  mixed  in  suitable 
proportions  for  a  melting  charge,  and  fused,  they  always  form  suffi- 
ciently good  electrical  conductors. 

1  Borchers'  Electric  Smelting  and  Refining  (1897  Ed.).     Figs.  157,  158,  and 
165. 

2  This  convenient  term  for  "a  substance  used  because  of  its  property  of  offering 
resistance  to  the  passage  of  an  electric  current,"  was  suggested  by  F.  A.  J.  Fitz- 
Gerald,  Electrochemical  Industry,  vol.  ii,  p.  490,  to  avoid  attaching  two  meanings 
to  the  word  "resistance." 


CONSTRUCTION  AND  DESIGN  87 

The  conductivity  of  molten  slags  enables  continuous  smelting  fur- 
naces to  be  operated  electrically,  although  the  ore  fed  into  the  fur- 
nace may  be  non-conducting.  The  furnace  may  be  started  in  the 
first  place  by  means  of  an  arc  between  the  electrodes;  the  heat  of  the 
arc  melting  some  of  the  surrounding  material,  which  ultimately 
fills  the  space  between  the  electrodes  with  a  molten  conducting 
slag.  Heat  is  then  generated  by  the  passage  of  the  current  through 
the  slag,  more  ore  becomes  heated  and  melted,  and  after  a  time  the 
whole  crucible  of  the  furnace  becomes  thoroughly  heated  and  filled 
with  molten  slag  and  metal.  Another  way  of  starting  such  a  fur- 
nace is  by  placing  some  coke  between  the  electrodes.  The  coke, 
being  a  moderately  good  conductor,  soon  becomes  heated  by  the 
passage  of  the  current,  and  melts  the  surrounding  ore  charge.  The 
electrodes  are  then  pulled  further  apart  and  the  operation  goes  on 
as  described  above.  A  third  method  consists  in  pouring  some  molten 
slag  into  the  furnace  when  the  current  may  be  at  once  switched  on, 
and  the  furnace  will  soon  be  in  regular  operation. 

Although  the  ordinary  rocks  and  ore  minerals  are  very  poor  elec- 
trical conductors,  when  cold,  the  coke,  which  is  often  added  to  the 
charge  as  a  reducing  reagent,  is  a  fair  conductor,  and,  if  present  in 
sufficient  quantity,  will  render  the  charge  somewhat  conducting. 

The  writer  has  attempted  to  calculate  the  resistivity  of  the  melt- 
ing materials  in  the  fusion  zones  of  the  Heroult  and  Keller  ore 
smelting  furnaces,  and  also  of  molten  slags  themselves.  The  data 
available  were  very  unsatisfactory,  and  the  results  obtained  can 
only  be  taken  as  representing  in  the  roughest  wa-y  the  resistivities 
of  these  materials.  The  Heroult  and  Keller  smelting- zones  ap- 
pear to  have  a  resistivity  of  about  o.i  ohm  for  i  cu.  in.,  varying 
perhaps  from  about  0.05  to  0.15  ohm.1  The  resistivity  of  molten 
slag  is  about  i  or  2  ohms  for  i  cu.  in.2  In  the  Gin  and  Kjellin  steel 
furnaces,  the  resistivity  of  molten  iron  is  an  important  factor;  and 
this  is  very  small,  being  about  0.000,07  ohm  for  i  cu.  in.3 

1  Calculated  from  the  published  drawings  and  electrical  measurements  for 
these  furnaces,  assuming  that  the  resistivity  is  uniform  throughout  the  volume 
between  the  bottom  of  the  electrode  and  the  surface  of  the  melted  charge. 

2  The  resistivity  of  fused  salts  is  of  about  this  order,  see  J.  W.  Richards,  Con- 
duction in  fused  and  solid  electrolytes.     Trans.  Amer.  Electrochem.  Soc.,  vol. 
vii,  p.  71. 

8  G.  Gin  (Haanel's  European  Report,  1904,  p.  172),  gives  the  resistivity  of 
molten  pig-iron  as  2i6Xio~6  ohms  per  centimeter  cube  (  =  0.000,085  ohm  per 
inch  cube).  He  has  also  measured  the  resistivity  of  molten  pig-iron  at  1,300° 
C.  (Trans.  Electrochem.  Soc.,  vol.  viii,  p.  289),  and  finds  it  to  be  i6X  io~5  ohms 
per  cubic  centimeter  (  =  0.000,063  onm  per  inch  cube). 


88 


THE  ELECTRIC  FURNACE 


(3)  Furnaces  having  Special  Resisting  Cores. — -The  cores  or  re- 
sistors in  such  furnaces  are  usually  composed  of  carbon,  which,  in 
the  form  of  coke-powder,  for  example,  is  of  moderate  conductivity. 
It  therefore  allows  large  currents  to  flow,  and  at  the  same  time  has 
a  sufficient  electrical  resistivity  to  enable  fairly  high  voltages  to 
be  employed — even  when  the  cores  are  of  considerable  cross-sec- 
tion and  moderate  length.  The  resistivity  of  powdered  carbon 
depends  upon  the  fineness  of  grain,  as  well  as  upon  the  resistivity 
of  the  solid  material  from  which  the  powder  was  produced.  In  order 
to  obtain  uniform  heating,  it  is  advisable  to  sort  the  powder,  only 
using  particles  that  are  of  a  uniform  size;  under  such  conditions 
the  resistivity  increases  with  the  finen  ess  of  the  powder.  The  follow- 
ing resistivities  for  graphitized  coke-powder  have  been  calculated 
from  experiments  by  FitzGerald.1  The  resistivity  of  ordinary  coke- 
powder  would  probably  be  about  four  times  as  large. 

TABLE  VIII.— RESISTIVITY  OF  GRANULAR  GRAPHITIZED  COKE  , 
(Ohms  for  i  cu.  in.) 


Size  of  Grains 

Cold 

Red  hot 

Red  hot 

and  weighted 

Between  5  and  6  meshes  to  the  inch  . 

o  ^6 

o.  24 

O    I  < 

Between  3  and  4  meshes  to  the  inch  

0.29 

o.  19 

0.  II 

The  first  powder  had  been  passed  through  a  sieve  having  5  meshes 
to  the  linear  inch,  and  had  been  passed  over  a  sieve  of  6  meshes 
to  the  inch.  The  second  powder  had  been  passed  through  a  3- 
mesh  sieve  and  over  a  4-mesh  sieve.  The  resistivities  are  given 
for  the  cold  powder,  and  at  a  red  heat.  The  third  column  shows 
the  resistivity  of  the  red-hot  powder  when  a  weight  was  laid  upon 
it,  thus  making  a  better  electrical  contact  between  the  adjacent 
grains.  The  powder  was  placed  in  an  open  trough,  and  was  only 
4  in.  in  depth;  it  would,  therefore,  be  more  lightly  packed  than  in 
the  core  of  a  full-sized  furnace.  The  figures  in  the  last  column  would 
consequently  more  nearly  represent  regular  furnace  conditions.  The 
figures  are  given  for  i  cu.  in.,  as  inches  are  still  more  frequently  used, 
in  this  country,  than  centimeters;  to  convert  to  centimeter  resis- 
tivities, multiply  by  2.54  —  the  number  of  centimeters  in  i  in. 
Solid  rods  of  carbon  (amorphous  or  graphitized)  are  sometimes 
used  as  resistors,  as  in  Borchers'  resistance  furnace,  Fig.  16,  or  in 
Acheson's  siloxicon  furnace,  Fig.  120.  The  resistivity  of  rods  of 
carbon,  such  as  are  used  for  electric  lighting  and  furnace  electrodes, 


1  Francis  A.  J.  FitzGerald.     Electrochemical  Industry,  vol.  ii  (1904),  p.  490. 


CONSTRUCTION  AND  DESIGN 


89 


and  of  the  graphitized  electrodes,  is  very  much  less  than  that  of  the 
same  material  in  the  form  of  powder.  The  following  are  approxi- 
mate values: 

TABLE  IX.— RESISTIVITY  OF  SOLID  CARBON  » 
(Ohms  for  i  cu.  in). 


Cold 

Hot 

Amorphous 

o  00124—0.00163 

about  o.ooio 

Graphitic  

O  .  OOO3  2-O  .  OOO42 

about  0.00025 

In  this  table,  " amorphous"  refers  to  the  ordinary  carbon  electrode, 
or  arc-light  carbon;  while  "graphitic"  refers  to  the  graphitized 
electrodes.  The  word  "hot"  refers  to  electric-furnace  tempera- 
tures, such  as  2,000°  C.,  or  3,000°  C.,  and  it  will  be  obvious  that 
only  approximate  values  can  be  given. 

The  smaller  values  under  the  heading  "cold"  were  determined 
by  Mr.  P.  M.  Lincoln,  of  the  Niagara  Falls  Power  Co.,  on  rods  of 
about  1.6  sq.  in.  cross-section,  and  about  12  in.  long.  They  are 
published  by  the  Acheson  Graphite  Company  in  their  pamphlet 
on  Acheson  Graphite  Electrodes.  The  larger  values  are  taken 
from  a  paper  by  Messrs.  FitzGerald  and  Forssell2  and  represent 
a  large  number  of  experiments  on  electrodes  of  4X4  in.  section, 
and  from  40  in.  to  93  in.  in  length.  The  values  under  the  heading 
"hot"  do  not  represent  actual  experiments.  Unfortunately  the 
experiments  of  Messers.  FitzGerald  and  Forssell  were  not  continued 
to  sufficiently  high  temperatures  to  give  much  information  about 
the  resistivity  at  electric-furnace  temperatures,  but  as  far  as  they 
go  they  indicate  a  more  rapid  decrease  of  resistivity  with  tempera- 
ture in  the  graphite  than  in  the  amorphous  carbon,  as  shown  in 
Table  X.  The  experiments  were  made  at  the  works  of  the  National 
Carbon  Company. 

TABLE  X.— RESISTIVITY  OF  AMORPHOUS  AND  GRAPHITIC  CARBON 
Experiments  by  FitzGerald  and  Forssell 


Resistivity,  ohms  for  i-in.  cube 


j.empera,iure 

Amorphous 

Graphitic 

10°  C. 
61°  C. 
109°  C. 

0.00163 
0.00160 

0.000416 
0.000387 
o  0003^6 

185°  C. 
282°  C. 
390°  C. 

0.00158 
0.00153 

0.000338 

466°  C. 

0.00150 

1  Compare  with  the  figures  given  in  Table  XI. 

2  FitzGerald  and  Forssell,  Trans.  Amer.  Electrochem.  Soc.,  vol.  xi  (1907),  p.  317. 


90  THE  ELECTRIC  FURNACE 

The  results  quoted  are  from  an  amorphous  carbon  electrode, 
4X4  in.  section  and  73  in.  long,  and  from  an  Acheson  graphitized 
electrode  4X4  in.  section  and  40  in.  long. 

In  large  electric  furnaces  having  a  special  resistor,  some  form  of 
carbon  is  generally  used  for  this  purpose,  but  in  smaller  furnaces 
there  are  a  number  of  other  materials  that  may  be  employed,  par- 
ticularly when  only  a  moderate  temperature  is  required. 

First  there  are  certain  metals  and  alloys  which  are  used  as  wires 
to  make  heating  coils  in  laboratory  furnaces.  Platinum,  iron  and 
nickel  are  the  most  commonly  used  metals,  while  nichrome  (steel 
containing  nickel  and  chromium),  German  silver  and  Monel  metal 
(70  per  cent,  nickel  30  per  cent,  copper)  are  examples  of  alloys  used 
for  this  purpose.  Silicon,  on  account  of  its  high  melting-point  and 
electrical  resistivity,  makes  a  suitable  resistor,  and  rods  of  silun- 
dum  (carbon  converted  into  carborundum),  are  used  for  resistors 
in  high- temperature  furnaces.  Carborundum  itself  is  a  non-con- 
ductor when  cold,  but  the  rods  of  silundum  have  a  carbon  core 
which  carries  the  current,  and  after  the  rod  has  become  heated, 
the  carborundum  becomes  an  electrical  conductor  and  serves  as  a 
resistor.  The  "Nernst  Earths"  are  refractory  oxides  which  are 
non-conductors  when  cold,  but  conduct  when  red  hot  and  are  used 
for  electric  lamps,  being  heated  in  the  first  place  by  an  auxiliary 
source  of  heat.  These  earths  have  been  used  as  resistors  in  ex- 
perimental electric  furnaces.1  Being  very  refractory  they  can  be 
used  for  high- temperature  furnaces  whenever  it  is  undesirable  to 
use  carbon. 

In  the  following  table  are  collected  the  resistivities  of  a  number 
of  substances  that  may  be  used  as  resistors,  approximate  values 
being  given  for  the  resistivity  at  furnace  temperatures  as  well  as 
when  cold. 

NOTES  ON  TABLE  XI 

F.  Fulton,  Principles  of  Metallurgy. 

L.  Landolt,  Bornstein  physikalisch-chemische,  Tabellen,  1905. 

H.  Hering,  Electrode  Calculations.     See  Table  XIII. 

D.H.  Driver-Harris  Wire  Co.,  "Resistance  Materials." 

G.  G.  Gin,  Dr.  Haanel,  European  Report. 
F.G.  FitzGerald.    See  Table  VIII. 

S.  Stansfield,  Tests  made  in  the  author's  laboratory. 

T.  Tucker,    Trans.  Electrochem.  Soc.,  xii,  p.  171  * 

(  )  Figures  in  brackets  are  only  approximate. 

Calc.  Calculated  from  the  cold  values,  using  a  temperature  coefficient. 

1  Harker  tube  furnace,  Fig.  70. 


CONSTRUCTION  AND  DESIGN 


91 


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H 


u  o  v- 

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CJ  CJ 

o      o 


CJ  CJ 


r 


c 

CJ     §  O 


V)  00 

H       O 

O     O 


O     O     O 


o    o 
o   o 


i 


000000 


§  o  8 


O\  *O     t-~ 

^f      M       IO 

000 


o    o    o     •     •    o    o 


o    o    o    o    o    o 


^•^s          oO     O     ^" 
H  oir^t^OiOwOvOO 

8OOOMHMMCOO 
O     O     O^    O^    O^    O^    O^    O^    M^ 

o"   o^o'o'o'o^o'o^So 


o    oooooooooooo 


8  8 


o    oooooooooooo 


w  w        o 


^   PH*  t-i   fe    S   fe   P^' 


s 

i  Is 


s, 


o    ^  S  8  1    ji  S  8  ^  8  a  1 

Cj       <!z;pq^       pHLjO^OCJO 


ii 


92  THE  ELECTRIC  FURNACE 

THE  ELECTRICAL  RESISTIVITY  OF  FIRE-BRICKS  AT  HIGH 
TEMPERATURES  * 

Fire-bricks,  and  most  refractory  materials,  are  electrical  insu- 
lators when  cold,  but,  as  is  well  known,  at  furnace  temperatures 
they  are  partial  conductors,  and  this  affects  their  use  in  the  con- 
struction of  electrical  furnaces.  Hardly  any  data  have  been  pub- 
lished, however,  with  regard  to  the  electrical  resistivity  of  these 
materials  at  high  temperatures.  This  matter  is  of  some  impor- 
tance with  regard  to  the  design  of  electric  furnaces,  because  in  many 
of  these  there  is  an  opportunity  for  the  electric  current  to  flow  in 
part  through  the  heated  walls  of  the  furnace  instead  of  merely 
through  the  resistor,  or  through  the  charge  which  is  to  be  heated. 
Such  leakage  may  not  be  harmful  in  some  instances,  but  in  others 
it  may  tend  to  overheat  and  melt  the  brickwork,  and  in  any  case 
it  is  desirable,  when  designing  a  furnace,  to  know  the  probable 
magnitude  of  this  effect. 

Another  instance  where  a  knowledge  of  the  resistivity  of  furnace 
materials  would  be  desirable  is  in  designing  a  furnace  like  the 
Rodenhauser  induction  furnace,  where  the  current  is  supplied  to 
"pole  pieces"  and  passes  through  a  wall  of  refractory  material  before 
entering  the  steel  in  the  furnace.  In  such  a  case  it  would  be  desir- 
able to  know  the  electrical  resistance  of  the  plate  of  refractory  mate- 
rial separating  the  "pole  piece"  from  the  contents  of  the  furnace. 

The  following  experiments  were  made  by  Messrs.  McLeod  and 
McMahon  as  an  undergraduate  research  in  the  Metallurgical 
Department  of  McGill  University. 

The  materials  tested  were  Caledonia  fire-brick,  Star  silica-brick, 
magnesite-brick  and  chrome-brick,  the  last  three  being  obtained 
from  Messrs.  Harbison- Walker.  Cylinders  2  in.  in  diameter  and 
about  21/2  in.  long  were  cut  from  these  bricks,  the  length  of  the 
cylinder  being  the  thickness  of  the  brick.  These  cylinders  were 
supported,  as  shown  in  Fig.  34,  between  discs  of  graphite  of  the 
same  diameter,  and  contact  was  made  between  the  cylinders  and 
the  disc  by  means  of  a  paste  of  graphite  and  glue.  In  the  first 
few  experiments  the  apparatus  was  heated  in  a  gas  furnace,  but 
this  method  of  heating  was  found  unsatisfactory,  and  electrical 
heat  was  employed  in  the  apparatus  shown. 

In  the  figure,  F  is  the  test-cylinder  and  E  E  are  carbon  elec- 
trodes supplying  the  testing  current.  Direct  current  was  used  at 

1  A.  Stansfield,  D.  L.  McLeod,  J.  W.  McMahon,  Trans.  Am.  Electrochem. 
Soc.,  xxii,  1912,  p.  89. 


CONSTRUCTION  AND  DESIGN 


93 


no  volts,  with  a  rheostat  of  lamps  to  avoid  the  danger  of  accident 
and  to  reduce  the  current  flowing  through  the  test-piece  when 
the  resistance  became  low.  The  current  passing  through  F  was 
read  on  an  ammeter.  To  avoid  any  possible  error  through  polari- 
zation, due  to  the  use  of  a  direct  current,  a  reversing  switch  was 
used  so  that  the  current  could  be  reversed  in  the  test-piece  before 
taking  the  reading.  The  testing  current  was  supplied  to  the  car- 
bon electrodes  E  E  by  metal  clamps,  and  the  voltage  was  read  on 
a  voltmeter  connected  to  the  same  clamps.  The  resistance  meas- 
ured includes  the  resistance  of  the  carbon  electrodes  E  E,  which 
would  be  about  0.13  ohm,  and  would  represent  a  correction  of  0.4 
ohm  to  be  deducted  from  the  resistivities  stated  in  the  table.  These 


H   IE 


FIG.  34. — Furnace  for  testing  electrical  resistivity. 

figures  also  include  the  contact  resistances  between  the  ends  of  the 
test-pieces  and  the  carbon  discs.  These  resistances  were  made  as 
small  as  possible,  and  were  probably  a  small  fraction  of  the  whole 
resistance.  The  method  adopted  is  somewhat  crude  from  an 
electrical  point  of  view,  but  it  was  considered  essential  that  the 
construction  of  the  apparatus  should  be  as  simple  as  possible  in 
view  of  the  high  temperature  at  which  the  tests  had  to  be  made. 
The  test-piece  was  enclosed  in  a  graphite  crucible  D  in  such  a  way 
that  the  testing  current  could  not  be  -short-circuited  through  the 
graphite  crucible  or  through  the  heated  portion  of  the  furnace 
walls.  A  thermo-couple  pyrometer  H,  in  a  silica  tube,  G,  was  used 
to  indicate  the  temperature  of  the  test-piece.  The  furnace  was 


94 


THE  ELECTRIC  FURNACE 


filled  around  the  crucible  with  broken  graphite  varying  from  1/2 
in.  to  1/4  in.  in  size.  Alternating  current  was  supplied  by  means 
of  water-cooled  electrode  holders  /  to  the  i  i/2-in.  electrodes  C  C, 
having  blocks  of  graphite  on  their  ends  for  distributing  the  current. 
From  300  to  600  amperes  at  about  30  volts  were  used  for  heating 
the  furnace,  and  any  temperature  up  to  about  1700°  C.  could  easily 
be  obtained.  The  voltage  of  the.  test  current  was  about  100  at 


1600 


'°80Q°C900'  1000°  1100'  1200"  1500*  MOO'  1500°  1600*' 
FIG.  35. — Electrical  resistivity  at  high  temperatures. 

low  temperatures,  and  dropped  to  5  or  10  volts  at  the  highest  tem- 
peratures. The  ammeter  could  be  read  to  o.oi  ampere,  and  the 
largest  current  measured  was  about  1.2  amperes  at  about  5  volts. 

The  thermo-couple  was  of  platinum,  and  platinum  with  10  per 
cent,  of  rhodium.  It  was  calibrated  at  the  melting-point  of  copper, 
1083°  C.,  and  the  melting-point  of  the  couple  itself,  which  was  taken 
as  1775°  C.1  The  calibration  curve  was  assumed  to  be  a  straight 
line  between  these  points. 

1  The  melting  temperature  of  platinum  is  now  considered  to  be  1755°  C.  and 
the  temperatures  given  in  Table  XII  may  therefore  be  a  little  too  high. 


CONSTRUCTION  AND  DESIGN 


95 


The  results  obtained  from  these  experiments  have  been  tabu- 
lated, and  are  also  shown,  in  part,  in  Fig.  35.  It  will  be  seen  from 
these  that  at  1500°  C.  the  resistivity  of  all  these  materials  is  some- 
what low.  Comparing  the  results  for  the  different  materials,  it 
will  be  noticed  that  the  silica-  and  magnesia-bricks,  which  approxi- 
mate most  closely  to  pure  oxides,  retain  their  insulating  qualities 
to  high  temperatures,  no  considerable  current  being  obtained  be- 
low about  1300°  C.  The  fire-brick  and  chrome-brick,  on  the 
other  hand,  which  are  more  complex  in  composition,  begin  to  con- 
duct at  considerably  lower  temperatures.  At  the  highest  tempera- 
tures observed,  about  1550°  C.,  the  resistivities  of  all  the  bricks 
tested  were  about  the  same,  being  about  25  ohms  for  a  i-cm.  cube. 
The  results  obtained  for  the  chrome-brick  are  very  peculiar,  and 
should  be  regarded  as  subject  to  correction. 

TABLE  XII.— ELECTRICAL  RESISTIVITY  OP  FIRE-BRICK 
(Ohms  for  i-cm.  cube) 


Temperature 

Caledonia 
fire-brick 

Silica-brick 

Magnesite- 
brick 

Chrome- 
brick 

600°  C 

21  OOO 

700° 

17  OOO 

800° 

13  ooo 

2,800 

000° 

o  ooo 

760 

000° 

6  600 

420 

1  00° 

A    AOO 

4.30 

200° 

2  3OO 

4t»O 

,3oo° 

,400° 

COO° 

1,300 

690 

280 

9,700 
2,400 

7IO 

6,200 

420 

ire; 

410 
320 

rc0° 

60 

22 

•2Q 

i.*6*° 

18 

2S 

ELECTRODES 

The  word  electrode  was  originally  applied  to  the  terminal  leading 
the  electric  current  to  or  from  an  electrolyte,  and  an  electrode  was 
known  as  anode  or  cathode  according  as  the  electric  current  entered 
or  left  the  electrolyte  by  means  of  it.  In  the  great  majority  of  elec- 
tric furnaces  no  electrolytic  action  takes  place,  but  the  conductors, 
usually  of  carbon,  by  means  of  which  the  current  is  led  from  the 
terminals  of  the  cables  to  the  heated  conducting  material  in  the 
furnace,  are  known  as  electrodes. 

Considered  in  this  sense,  the  function  of  an  electrode  is  to  convey 


A 

96  THE  ELECTRIC  FURNACE 

the  electric  current  from  the  electric  cables,  which  are  necessarily 
at  a  low  temperature,  to  the  heated  resistor  or  arc  in  the  furnace. 
An  electrode  must  therefore  be  an  electrical  conductor  which  can 
be  heated  to  a  high  temperature.  An  efficient  electrode  will  be  a 
good  electrical  conductor  so  as  to  use  little  electrical  energy,  and 
a  poor  conductor  of  heat  so  as  to  waste  little  heat  by  conduction 
from  the  furnace.  An  electrode  should  also  be  affected  as  little  as 
possible  by  the  heated  contents  of  the  furnace,  and  should  not  be 
of  such  a  nature  as  to  affect  unfavorably  the  product  to  be  obtained. 

With  scarcely  any  exception,  electrodes  are  made  of  metal  or  of 
carbon;  the  great  majority  being  of  carbon.  The  fusibility  of 
metals  does  not  prevent  their  use  as  electrodes,  as  they  can  be 
contained  in  channels  so  as  to  retain  their  shape  when  melted. 
Metal  electrodes  can  be  used  in  furnaces  containing  a  molten  bath 
of  the  same  metal,  or  in  furnaces  which  are  at  a  lower  temperature 
than  the  melting-point  of  the  metal.  In  most  other  cases  carbon 
electrodes  are  used. 

Carbon  electrodes  are  made  of  some  form  of  carbon  mixed  with 
tar  and  pitch,  molded,  and  baked  to  drive  off  the  volatile  matter. 
Retort  carbon  and  petroleum  coke  have  been  largely  used  and  are 
still  employed  whenever  the  purity  of  the  electrode  is  important, 
as  in  many  electrolytic  operations.  In  electric  smelting,  however, 
mechanical  strength,  electrical  conductivity  and  cheapness  are 
more  essential  than  extreme  purity,  and  anthracite  is  now  generally 
used  in  the  manufacture  of  large  electrodes  for  this  purpose.1 

Electrodes  made  in  this  manner  can  be  graphitized  by  heating 
them  to  a  very  high  temperature  in  the  Acheson  graphite  furnace, 
if  they  contain  some  carbide- forming  substance  to  assist  the  conver- 
sion of  the  amorphous  carbon  into  graphite.  About  1.5  per  cent, 
or  2  per  cent,  of  hematite  would  serve  for  this  purpose,  while  if 
coke  or  anthracite  were  used  for  making  the  electrodes,  the  impurities 
in  these  materials  would  enable  the  carbon  of  the  electrode  to  be 
graphitized.  The  iron  which  is  contained  in  the  hematite,  and  the 
silica  and  alumina  in  the  coke  or  anthracite,  effect  the  conversion 
of  the  carbon  into  graphite  and  are  finally  expelled,  by  volatilization, 
at  the  extremely  high  temperature  of  the  furnace;  leaving  the  elec- 
trodes composed  of  compact  graphite.  Molten  iron  has  the  property 
of  dissolving  carbon,  which  separates  from  the  iron  as  graphite  on 
cooling;  but  it  is  difficult  to  understand  how  so  small  a  proportion 

1  R.  Turnbull,  "Furnace  Electrodes  Practically  Considered."  Trans.  Am. 
Electrochem.  Soc.,  vol.  xxi  (1912),  p.  397. 


CONSTRUCTION  AND  DESIGN  97 

of  iron,  or  other  substance,  can  change  the  whole  electrode  into 
graphite. 

Graphitized  electrodes  have  the  advantage  of  purity,  good  con- 
ductivity, and  great  resistance  to  oxidation.  Their  purity  renders 
them  very  suitable  for  operations  like  the  production  of  aluminium, 
in  which  the  electrode-ash  enters  the  electrolyte,  and  contaminates 
the  resulting  metal.  The  characteristic  resistance  of  these  elec- 
trodes to  oxidation  reduces  their  consumption,  and  their  good 
conductivity  has  a  similar  effect,  since  smaller  electrodes  can  be 
employed.  Graphitized  electrodes  are  largely  used  for  electrolysis, 
but  in  electric  smelting  furnaces,  cheaper  ones  made  of  coke,  or 
anthracite,  and  tar  have  usually  been  employed;  while  in  some  cases 
the  coke,  forming  part  of  the  furnace  charge,  has  been  utilized  for 
leading  in  the  current;  electrical  contact  being  made  through  the 
charging  hoppers. 

Approximate  figures,  for  the  resistivity  of  carbon  and  graphite 
electrodes,  have  already  been  given.  By  means  of  these,  it  is  easy 
to  calculate  the  drop  of  voltage  that  would  be  produced  in  electrodes 
of  a  certain  length  and  cross- section,  by  any  particular  current. 
The  cross- section  of  an  electrode  is  largely  determined  by  the 
amount  of  current  to  be  carried.  The  current- density,  or  the  number 
of  amperes  per  square  inch  of  cross- section  of  the  electrode,  differs 
considerably  in  different  types  of  furnaces  and  for  different  kinds 
of  electrodes,  being  much  higher  in  graphitized  electrodes  than  in 
the  ordinary  variety.  The  large  carbon  electrodes  used  in  the 
Heroult  and  Keller  furnaces  carry  about  20  amperes  per  square 
inch,  while  small  round  carbon  electrodes  and  large  graphite 
electrodes  carry  more,  up  to  about  100  amperes  per  square 
inch.  Moissan  used  currents  up  to  200  or  even  700  amperes 
per  square  inch,  in  small,  ungraphitized  electrodes,  but  this 
would  be  far  too  high  for  commercial  work,  as  the  carbons 
would  become  red  hot  and  would  rapidly  waste  away,  and  the 
consumption  of  electrical  energy,  in  the  electrode,  would  be  too 
high  to  be  tolerated.  The  loss  by  oxidation,  of  the  exposed  part 
of  an  electrode,  can  sometimes  be  prevented  by  a  system  of  water 
jackets,  as  in  the  Heroult  steel  furnace,  Fig.  93. 1 

In  all  furnaces  in  which  electrodes  are  used  there  are  certain 

1  F.  M.  Becket  proposes  to  prevent  the  oxidation  and  destruction  of  carbon  or 
graphite  electrodes,  by  surrounding  them  with  water-cooled  jackets  at  the  point 
where  they  enter  the  furnace.    U.  S.  patent  855,44 1 ,  see  Electrochemical  Industry, 
vol.  v,  p.  279. 
7 


98  THE  ELECTRIC  FURNACE 

losses  of  energy  caused  by  the  electrodes,  and  usually  also  losses  of 
electrode  material.  These  losses  can  be  made  a  minimum  by  a  care- 
ful attention  to  the  material  and  dimensions  of  the  electrodes.  The 
nature  of  the  surrounding  bodies  is  of  great  importance  in  regard 
to  the  loss  of  electrode  material;  thus  a  carbon  electrode  wastes 
rapidly  when  immersed  in  a  slag  rich  in  iron  oxide,  or  if  exposed  to 
air  when  red  hot. 

Dimensions  of  Electrodes. — In  deciding  on  the  dimensions  of 
electrodes  we  have  to  consider  the  following  points: 

1.  The  essential  length  of  the  electrode,  as  determined  for  exam- 
ple by  the  thickness  of  the  furnace  wall. 

2.  The  number  of  amperes  of  current  to  be  carried. 

3.  The  conductivity,  for  electricity  and  for  heat,  of  the  material 
of  the  electrode. 

4.  The  temperature  of  the  furnace. 

5.  The  cross-section  of  the  electrode.     This  can  be  determined 
from  the  other  data. 

It  will  be  obvious  that  an  electrode  of  small  cross-section  will 
cause  large  losses  through  its  resistance  to  the  passage  of  the  elec- 
tric current,  and  that  one  of  large  cross-section  will  cause  large  losses 
by  conducting  heat  from  the  furnace  to  the  electrode  holders;  this 
loss  depending  also  on  the  temperature  of  the  furnace.  The  effect 
of  the  cross-section  on  the  loss  of  electrode  material  is  not  so  obvious, 
as  a  stout  electrode  will  have  a  larger  area  over  which  wasting  by 
solution  or  volatilization  can  take  place,  but  on  the  other  hand  its 
temperature  will  usually  be  lower  and  therefore  the  loss  per  square 
inch  of  surface  will  be  less.  Moreover,  although  the  loss  of  weight 
may  be  greater  with  the  stouter  electrode  the  loss  of  length  will 
certainly  be  less,  and  this  may  be  of  more  importance  in  regard  to 
economy  of  working  than  a  small  difference  in  the  rate  of  loss  of 
weight. 

The  most  satisfactory  course  will  be  to  determine  the  cross-sec- 
tion producing  the  least  loss  of  energy,  and  then  to  modify  this  if 
necessary  in  view  of  economy  of  electrode  material  and  convenience 
of  operation. 

Until  recently  it  was  generally  supposed  that  the  cross-section 
of  an  electrode  should  be  simply  related  to  the  amount  of  current  to 
be  carried,  giving  a  definite  current-density  for  any  particular  elec- 
trode material,  as  is  usual  in  calculating  the  size  of  copper  wires 
or  cables.  It  was  noted  however  that  large  electrodes  were  made  to 
have  a  much  smaller  current-density  than-small  ones,  but  this  was 


CONSTRUCTION  AND  DESIGN  99 

attributed  entirely  to  the  smaller  conductivity  of  the  larger  sizes  of 
carbon  electrodes. 

Dr.  Carl  Hering  published  in  1909  a  mathematical  treatment 
of  this  problem,  based  on  certain  assumptions  which  are  nearly 
correct  in  most  cases,  which  shows  that  it  is  not  the  cross-section 
itself,  but  the  ratio  of  cross-section  to  length,  which  should  be  propor- 
tionate to  the  amount  of  current  flowing.  Many  other  interesting 
relationships  are  established,  as  shown  in  the  following  "laws  of 
electrode  losses"  which  are  quoted  directly  from  Dr.  Hering's 


paper. l 


Laws  of  Electrode  Losses 


a.  The  combined  loss  through  the  cold  end  of  an  electrode  is 
equivalent  to  the  sum  of  the  loss  by  heat  conduction  alone2  (when 
there  is  no  current)  and  half  the  C2R  loss.3 

b.  The  combined  loss  will  be  least  when  the  loss  by  heat  conduc- 
tion alone  is  made  equal  to  half  the'  C2R  loss;  the  total  loss  will 
then  be  equal  to  the  C2R  loss  and  no  heat  will  be  conducted  from 
the  interior  of  the  furnace. 

c.  This  minimum  loss  is  dependent  only  on  the  material,  current, 
and  temperature,  but  not  on  the  absolute  dimensions;  it  merely 
fixes  the  relation  of  the  cross-section  to  the  length,  but  leaves  a 
choice  of  either;  hence, 

d.  For  economy  of  electrode  material  the  length  should  be  made 
as  short  as  practical  considerations  permit. 

e.  For  each  material  there  is  a  definite  minimum  loss  of  electrode 
voltage  which  depends  only  on  the  temperature  and  is  independent 
of  the  dimensions  or  the  normal   current  for  which  the  furnace 
is  designed;  hence, 

j.  The  best  possible  electrode  efficiency  for  any  material  may 
be  determined  from  the  total  voltage  of  the  furnace  and  this  mini- 
mum voltage  due  to  the  material  and  the  temperature,  and  is  inde- 
pendent of  the  dimensions. 

g.  The  temperatures  indicated  by  the  heat  gradient  of  the  com- 
bined flow  are  equal  to  the  sums  of  those  of  the  individual  flows. 

1  Carl  Hering,  Trans.  Am.  Electrochem.  Soc.,  vol.  xvi,  1909,  p.  265;  Elec- 
trochem.  and  Metall.  Industry,  vol.  vii,  1909,  p.  442. 

2  The  rate  at  which  heat  would  flow  along  the  electrode,  from  the  furnace 
to  the  holder,  if  no  electric  current  were  supplied. 

3  "  C2R  loss."    The  rate  of  heat  production  in  the  electrode  due  to  the  passage 
of  the  electric  current. 


100  THE  ELECTRIC  FURNACE 

These  laws  depend  on  the  assumptions  that  the  electrodes  have 
a  uniform  cross-section  throughout  their  length,  that  there  is  no 
transfer  of  heat  between  the  electrodes  and  the  furnace  walls,  and 
that  the  conductivities  for  heat  and  electricity  are  linear  functions  of 
the  temperature. 

The  mathematical  proof  of  these  laws  is  given  in  the  same  paper. 
The  calculation  of  electrode  dimensions  and  electrode  efficiencies 
can  be  readily  made  with  the  aid  of  the  following  formulae  given  by 
Dr.  Hering.1 

Symbols  Used  in  the  Formulae 

S  =  Cross-  section  of  electrode  in  square  inches. 

L  =  Length  of  electrode  in  inches. 

C  =  Current  in  amperes. 

W  =  Heat  generated  in  the  electrode,  expressed  in  watts. 

H  =Flow  of  heat,  in  watts,  due  to  heat-conductivity  of  electrode, 

if  no  electric  current  were  flowing. 

h  =  Flow  of  heat,  in  watts,  from  the  furnace  to  the  electrode. 
X  =  Flow  of  heat,  in  watts,  leaving  cold  end  of  electrode. 
T  =  Difference  of  temperature,  in  Centigrade  degrees,  between  the 

ends  of  the  electrode. 
r  =  Electrical  resistivity  of  the  electrode  material  in  ohm,  inch 

cube  units. 
k   —  Thermal    conductivity    of    the  electrode  material,  in  watt, 

inch  cube  units. 

E  =  Voltage  between  ends  of  electrode,  or  watts  per  ampere. 
e    ="  Electrode  voltage." 
s   ="  Specific  cross-section"  in  square  inches. 
In  general: 

117 

d) 


where       H  =  kTJj  0) 


and  TF  =  C2*  (4) 

1  Dr.  Carl  Hering,  Am.  Inst.  Elect.  Engrs.,  vol.  xxix,  1910,  p.  285. 


CONSTRUCTION  AND  DESIGN  101 

W 
The  loss  of  heat,  X,  will  be  a  minimum  when  H=  —  '     Call  this 

minimum  loss  mX,  then: 

(from  i,  2,  3  and  4)       (5) 


=  (fr°m  3  and  5) 


(8) 

(9) 
'  (10) 


The  "specific  section,"  s,  is  the  cross-section  of  an  electrode  carrying 
i  ampere,  having  a  length  of  i  in.  and  a  difference  of  temperature 
of  i°  C.  between  its  ends.  It  depends  simply  on  the  conductivity  for 
heat  and  electricity  of  the  electrode  material.  The  cross-section 
in  any  particular  case  can  be  obtained  from  this  by  the  formula: 

S-*™ 

VT 

The  "electrode  voltage,"  e,  also  depends  simply  on  the  conductivity 
for  heat  and  electricity  of  the  electrode  material.  It  represents 
the  voltage  between  the  ends  of  an  electrode,  proportioned  for 
minimum  loss,  and  having  a  difference  of  temperature  of  i°  C.  be- 
tween its  ends.  The  actual  voltage  between  the  ends  of  any  suitably 
proportioned  electrode  is  obtained  by  multiplying  this  "electrode 
voltage"  by  the  square  root  of  the  temperature  drop  in  Centigrade 
degrees. 

In  order  to  use  these  formulae  for  the  calculation  of  the  dimensions 
of  electrodes  we  must  know  the  conductivities  for  heat  and  for 
electricity  of  the  electrode  materials,  and  that  not  only  at  the 
ordinary  temperatures,  but  at  all  temperatures  up  to  those  of  the 
electric  furnace.  Electrical  conductivities  are  easily  measured, 
and  are  known  accurately  for  ordinary  temperatures,  but  less  exactly 
at  high  temperature.  Heat  conductivities  are  difficult  to  measure 
at  any  temperature.  Dr.  Hering1  has  shown,  how  to  obtain  with 

1  Carl  Hering,  A  New  Method  of  Measuring  Mean  Thermal  and  Electrical 
Conductivities  of  Furnace  Electrodes.  Trans.  Am.  Electrochem.  Soc.,  vol. 
xvi,  1909,  p.  317. 


102 


TEE  ELECTRIC  FURNACE 


comparative  ease  values  for  the  conductivities  of  electrodes,  for 
both  heat  and  electricity,  which  shall  fairly  represent  the  effective 
mean  values  between  the  hot  and  cold  ends  of  the  electrode.  The 
test  is  made  by  providing  an  electrode  A,  B,  Fig.  36,  with  a  water- 
cooled  terminal  at  each  end,  jacketing  it  as  perfectly  as  possible 
throughout  its  length  in  refractory  material  and  applying  a  pyro- 
meter, P,  at  its  middle  point.  A  large  electric  current  is  passed 
through  the  electrode  so  as  to  heat  it.  The  middle  section  of  the 
electrode  will  attain  the  highest  temperature  and  this  will  be  indi- 
cated by  the  pyrometer  which  is  placed  there.  When  stable  condi- 


FIG.  36.— Electrode  testing. 

tions  have  been  reached,  the  rod  will  represent  two  electrodes,  am, 
and  mb,  having  their  lengths  and  cross- sec  tions  properly  proportioned 
for  that  particular  current  and  temperature;  the  latter,  indicated 
by  pm,  represents  the  temperature  of  the  furnace.1  Each  half  of  the 
rod  represents  a  correctly  proportioned  electrode  because  no  heat 
passes  in  or  out  through  the  hot  end,  that  is  through  the  middle 
section  of  the  double  electrode,  and  all  the  heat  generated  in  the 
electrode  finds  its  way  out  through  the  cold  end;  conditions  which 
have  been  shown  analytically  to  hold  in  the  case  of  perfectly  pro- 
portioned electrodes. 

Having  measured  the  different  temperature  drops  between  the 

- 1  Really  the  difference  of  temperature  between  the  hot  and  cold  ends  of  the 
electrodes. 


CONSTRUCTION  AND  DESIGN 


103 


hot  and  cold  ends  produced  by  different  amounts  of  electric  current, 
and  having  also  the  voltages  across  the  terminals,  we  can  calculate 
the  correct  mean  values  of  the  thermal  conductivity,  electrical 
conductivity,  electrode  voltage  and  specific  section  for  each  current 
or  temperature  drop  from  the  foregoing  formulae.  Using  the  same 
letters  we  may  let  the  length  ab  =  2L,  and  the  voltage  between  a 
=  2E.  Then: 


ECL, 
2TS 


SE 
CL 


CL 

The  values  so  obtained  for  k  and  r  represent  the  effective  means 
of  the  thermal  conductivity  and  the  electrical  resistivity  between 


FIG.  37. — Electrode  testing. 

the  hot  and  cold  ends  of  the  electrodes  for  the  respective  drops  of 
temperature.  For  the  design  of  electrodes  we  only  need  the  values 
of  e  and  5  the  electrode  voltage  and  specific  section. 

As  it  is  impossible  to  find  a  perfect  heat  insulator  for  jacketing 
the  electrode  in  the  above  experiments,  a  number  of  similar  rods  of 
electrode  material,  cd,  ef,  etc.,  were  placed  around  the  test  electrode, 
ab,  as  shown  in  Fig.  37.  The  same  electric  current  was  passed 
through  each  rod  and  in  this  way  the  loss  of  heat  from  the  test  rod 
was  very  small,  as  the  surrounding  rods  were  very  nearly  at  the  same 
temperature  as  the  central  test  rod. 

The  electrodes  used  in  these  tests  were  of  amorphous  carbon, 
graphite,  iron  and  copper;  they  were  8  in.  long  between  the  points 


104 


ELECTRIC  FURNACE 


Thermal  conductivity; 
watts  per  degree  for 
i-in.  cube 
k 

•      10     <N     00    VO      Tf 

•    ON  ro  ro  -<t  in 

!      O      M      H      M      H 

o    10  o    o   in  o 

VO      -<t     <N      M      O*    t^ 
CO    ro    <"O    CO    f^     f>^ 

resistivity; 
i-in.  cube 
r 

H  VD   o  oo  H2  2. 

00    vo     10    -<t    ^    ^ 

t^    O     rf  vO     PO    ._. 
CO     CO     <N       H       <N      "      " 
CO    fO    ro    ro    co    £>    g3 

|5 

80    o    o   o   o 
O     0     0     O     O 

§§§§§§§ 

4-J       C/3 

II 

0     0     O     0     O     O 

0     O     0     O     0     O     O 

Specific  section; 
square  inches 
s 

\O       ON     <N 
ON    <"O     PO    N      M 
(N       <N       <N      fS      N 

0     0     0     O     O 
0     O     O     O     O 

i      t^     10    10     <N 
.     r^  00     ON    <N     £>    y« 
.     \o     >o     'O      t"-*    •ts*    r1* 
.     0     0     0     0     0     0 
•     O     0     0     0     O     O 

:   o'   6   6   6  o  o 

3  « 
2  % 
11* 

o    o    ON  o   in 

\O     co    ro    "*    IO 

10  \O    MD    ^    \O 
0     0     O     O     O 

•    CXD      CO    •*    l^ 

•      00        f^      10      Tf      «        H 

:  3-  ct  ct  3"  ?  o 

&  > 
w 

o   o    o   o   o 

•    o    o    o    o   o   o 

0> 

fl  s 

»fh 

0   1 
2 

u  u  u  u  o  o 

O        O        O        O        O        0 
O       O       H       <N       O       O 

vO     »0    •*    o     O 
«    ^>    00    e^   QJ 

U  CJ  U  U  U  0  0 

O        0        0        0        O        O        O 

O     O    0     0     ^    O     O 

ON      r)-      O)        MOO 
fN       Tj-    \O     OO      CO     Oj 

V 

„  % 

8  5 

nJ     cJ 

If 

u  u  u  u  o  o 

0        0        O        0        o        0 

O     O     w     <»     O     O 
MO      »O    •«*•    O      O 
CO    t^     Os    rj-    o 

1-T  rT 

U  U  U  U  U  U  0 

0        O        0        0        O        O        0 

O     O    O     O     rf    O     O 

<N      ON     Tf     <N      M      O      O 
CO     10    t^     ON    Tj-     O 
M      rT 

-3  -a 

o  'C 

!     !           ! 

£      4) 

Jl 

UJ      OJ      OJ      4i      4)      U      CJ 

d    d    d    d    d    d 

o    o    o    o    o    o 

XJ    X5    -Q    X5    Xi    42 
M  T<  Tj    H    N    H 

rt     o3     rt     rt     ed     c3 

u  u  u  u  u  u 

43      4H      J3      43      rd      43      43 

a  a  a  a  a,  a  a 
15   2   2   2   2   2   2 
O  O  O  O  O  O  O 

CONSTRUCTION  AND  DESIGN 


105 


>.  «-i 
.ti  c 

u   2i   « 
^    So  >o 
-T3     ^    3 

£  !?  ^  o"  o\  ON 

H      CO    ON  VO      w     t^    fO 

^    «  .d 
H    ^ 

M       <N       H       CS       M       M 

H      O      ON    ON    ON  00    00 

H 

00    00    00     co    ON 

vO     O     w     T^  OO    VO     ^O    O* 

5?   .S 
2     « 

o5  M    cT  co  <o  ^-  in 

0     O     O     O     O     O     O 

oooooooo 

°3     o 

8  8  8  8  8  1  § 

8  8  8  §  8  §  §  § 

ij     tn 

U     g 

a  -8 

O     O     O     O     0     O     O 

oooooooo 

l| 
§  1 

|s  " 

•    00     rf    t*»    co 
.      t^    vo  CO      M      CO    t*» 
•       M       (N       <N       CO     CO     CO 

80    o    o    o   o 
O     0     0     O     O 

M     ^    r^    H    oo    O    O 

.      <N       M      O       CO     CO    IO  VO 

s  & 

c^w 

•     0     O     O     O     O     O 

•    o    o    o    o    o   o   o 

Electrode 
voltage 
e 

•    800000 

•    odd    o   o   o 

:  8  8  8  8  8  1  § 

•    o    o    o    o    o   o   o 

.2 

n  a 

<u 

U  U  U  U  U  0  O 

0         O         O         O         O         0         O 
0     00       t~-     <N       10     O       O 
ON     CS       M      Tt     O      O 
(N     VO     00       l-^     CO     Oj 

W        M*      M" 

oooooooo 
O     t^»  oo     O     w     r^«    O     O 
ON  VO      t^    rl-     tfj    O      O 

M       CM       ^"      t^     CO     Ov 
M       M 

(U 
M 

8  5 
a   2 

1  1 

u  u  u  u  u  o  o 

o      o      o      o      o      o      o 
800    t^-    M     v>   O     O 
ON     M      M      Tt     O      O 

co    t^-    ON    cs     rt1    O 

oooooooo 

M       M       tO     *O    CO       ^     O 

nT   fT 

Electrode 
material 

C     fi     fl     fl     C3     C3     C- 

2222222 

^i_-^^i_uiuitJ 
OOOOOOOO 

UUUOUUUU 

V 


106  THE  ELECTRIC  FURNACE 

a  and  m,  and  5/8  in.  in  diameter.     The  cool  end  of  each  electrode, 
at  the  point  where  it  enters  the  holder,  was  kept  at  100°  C. 

The  results  of  these  tests1  are  reproduced,  in  a  condensed  form, 
in  Table  XIII.  The  electrode  voltages  and  other  properties  are 
the  mean  values  for  the  range  of  temperature  from  100°  C.  to  the 
"  furnace  temperature"  given  in  the  first  column  of  figures.  The 
figures  in  bold  face  do  not  represent  actual  observations  but  are  ob- 
tained by  exterpolation,  and  while  most  of  these  can  be  depended 
on  as  being  approximately  correct,  less  confidence  can  be  placed 
in  those  that  represent  the  properties  of  molten  iron  and  cop- 
per. The  electrical  resistivity  at  20°  is  included  for  the  purpose 
of  comparison. 

Examples.  —  As  an  example  of  the  use  of  this  table  it  will  be  of 
interest  to  determine  the  correct  dimensions  of  electrodes  of  car- 
bon, graphite,  iron  and  copper  for  a  furnace  which  is  kept  at  1,400° 
C.,  the  cool  end  of  the  electrode  being  at  100°  C.,  the  length  of 
electrode  20  in.  and  the  current  10,000  amperes.  The  copper  elec- 
trode would  of  course  be  molten  at  its  hot  end. 

The  cross-section  is  given  by  the  formula: 
CL 


10,000X20    _    200,000  S 

o  —  5        /  —  —  -  -  - 

Vi3°°  36-1 

_  0     200,000X0.022  . 

For  carbon,  S  =  —    —  7  -    —  =  122  sq.  in.  and  D=f  12.5  in.2 

T-  I--.L          o        200,OOOX.0073  . 

For  graphite,  S  =  —    -  —  7  --    —  =  40.5  sq.  in.  and  D*=J.i  in. 

_  200,000X0.0033  .  . 

For    iron,    S  =  —    —  7  -       -=  18.3  sq.  in.  and  £  =  4.8  m. 

200,000X0.0005  . 

ror  copper,  o  =  -  7  -    —=2.8  sq.  in.  and  D=i.g  in. 

The  drop  of  voltage  in  the  electrode  is  given  by  the  formula: 


For  carbon,  £=36.1X0.065  =  2.34  volts. 

and  the  loss  of  power  is  23.4  kw. 
For  graphite,  £  =  36.1X0.042  =  1.51  volts. 

and  the  loss  of  power  is  15.1  kw. 

1  Carl  Hering,  "The  Proportioning  of  Electrodes  for  Furnaces,"  Proc.  Am. 
Inst.  Elect.  Eng.,  vol.  xxix,  1910,  p.  285. 

z  D  is  the  diameter  of  the  electrode. 


CONSTRUCTION  AND  DESIGN 


107 


For  iron,  £  =  36.1X0.0128  =  0.46  volts. 

and  the  loss  of  power  is  4.6  kw. 
For  copper,  E  =  36.  i  X  0.0086  =  0.3 1  volts. 

and  the  loss  of  power  is  3.1  kw. 

The  results  of  these  calculations  are  shown  graphically  in  Fig. 
38.1  The  electrodes  of  each  material  are  drawn  to  scale  and  the 
black  line  below  each  indicates  the  relative  loss  of  power  caused 
by  that  electrode. 

The  total  loss  of  power  is  that  caused  by  all  the  electrodes,  and 

_.?^_  ._; 


Carbon 


Graphite 


o 


7.1  Dia. 


Iron 


O 


Copper 


I.  d" Diet. 


FIG.  38. — Equivalent  electrodes. 


the  "electrode  efficiency"  depends  on  the  number  of  electrodes 
and  the  voltage  of  the  furnace.  Thus  for  a  Heroult  steel  furnace, 
with  two  carbon  electrodes,  operating  at  100  volts,  the  total  elec- 
trode loss  would  be  2X23.4  =  46.8  kw.,  and  the  electrode  efficiency 

.  .        100  —  2X2.34 

would  be  —  =95-32  per  cent.2 

100 

1  This  figure  and  the  preceding  calculation  have  been  made  by  the  author. 
They  differ  slightly  from  a  calculation  and  illustration  given  by  Dr.  Heringin 
the  paper  quoted. 

2  The  Heroult  and  the  Girod  steel  furnaces  would  be  hotter  than  the  1400° 
C.  assumed  in  the  above  calculation,  and  the  electrode  losses  would  therefore  be 
somewhat  higher,  though  their  relative  values  would  be  unchanged. 

The  Hering  method  of  calculation  does  not  apply  directly  in  the  case  of  arc 


108  THE  ELECTRIC  FURNACE 

In  the  case  of  a  Girod  furnace  'having  one  carbon  electrode  and 
one  iron  electrode,  or  a  number  of  iron  electrodes  of  the  same  total 
cross- section,  the  loss  of  power  would  be  23.4  kw.  in  the  carbon 
electrode  and  4.6  kw.  in  the  iron  electrodes,  or  a  total  loss  of  28.0 
kw.  If  the  furnace  were  operated  at  60  volts,  there  being  only 

one  arc  in  this  case,  the  electrode  efficiency  would  be  — 

oo 

=  95.33  per  cent.,  which  is  practically  identical  with  the  Heroult 
furnace.  In  comparing  the  Heroult  and  the  Girod  furnace  it  will 
be  noticed  that  the  higher  voltage  of  the  Heroult  furnace,  which 
is  caused  by  having  two  arcs  in  series,  tends  to  give  a  higher  elec- 
trode efficiency  than  in  the  Girod  furnace,  which  has  only  one  arc 
in  series  and  therefore  a  lower  voltage.  One  of  the  electrodes  in 
the  Girod  furnace  is  of  iron,  and  the  higher  efficiency  of  the  iron 
electrode  quite  makes  up  for  the  loss  in  efficiency  due  to  the  lower 
voltage. 

Comparing  the  dimensions  of  electrodes  used  in  large  electric 
furnaces  with  the  dimensions  indicated  by  Dr.  Hering's  table  it 
will  be  found  in  general  that  electrodes  are  made  somewhat  stouter 
than  the  theory  demands.  This  will  cause  a  greater  loss  of  heat  by 
conduction,  but  the  electrode  will  last  longer  and  this  will  no  doubt 
more  than  offset  the  greater  loss  of  heat. 

ELECTRODE  HOLDERS 

These  are  employed  for  making  electrical  connection  between 
the  electrode  and  the  cable  which  supplies  the  electric  current. 
They  are  also  used  for  supporting  and  manipulating  movable  elec- 
trodes. The  holders  are  made  of  copper  or  bronze,  which  are  pref- 
erable on  account  of  their  good  electrical  conductivity,  or  of  iron 
or  steel,  which  are  cheaper  and  do  not  melt  so  easily  if  over-heated. 
It  is  not  easy  to  maintain  a  thoroughly  good  electrical  contact  be- 
tween the  holder  and  the  carbon  electrode,  because  the  electrodes 
and  their  holders  become  heated,  and  the  expansion  of  the  metal 
loosens  its  hold  on  the  carbon.  The  relatively  poor  conductivity 
of  carbon  makes  a  large  area  of  contact  desirable,  while  its  small  me- 

furnaces,  as  heat  escapes  laterally  from  the  electrodes  both  within  and  without 
the  furnaces.  The  part  of  the  electrode  within  the  furnace  can  be  neglected 
(being  considered  as  a  resistor)  but  the  air-cooled  or  water-cooled  part  outside 
the  furnace  will  introduce  complications.  If,  however,  the  electrode  is  measured 
from  the  holder  to  the  inside  of  the  furnace  wall,  the  formulae  can  be  used  with- 
out serious  error. 


CONSTRUCTION  AND  DESIGN 


109 


chanical  strength  renders  it  difficult  to  clamp  the  holder  sufficiently 
tightly  without  breaking  the  electrode.  In  addition  to  this,  the 
heat  of  the  furnace  tends  to  render  unworkable  any  bolts  and  nuts  or 
similar  mechanical  devices. 

Graphitized  electrodes  can  be  easily  machined  or  threaded,  and 
attached  in  this  way  to  the  holder;  but  for  electric  smelting  furnaces, 
electrodes  of  rectangular  cross-section  are  often  employed,  and 
these  are  secured  in  their  holders  by  bolting  or  clamping.  The 
electrodes  in  smelting  furnaces  are  usually  vertical,  in  order  to  be 
more  easily  manipulated,  and  are  suspended  by  a  chain,  so  as  to  be 


FIG.  39. — Water-cooled  holder  for  round  electrodes. 

easily  raised  or  lowered;  the  electric  cable  being  attached  directly 
to  the  electrode  holder. 

Electrode  holders  may  be  divided  into  terminal  holders,  which 
hold  the  electrode  by  the  end,  and  lateral  holders,  which  hold  the 
electrode  at  any  point  along  its  length.  Their  construction  depends 
also  upon  whether  they  are  intended  for  round  or  square  electrodes, 
and  whether  they  are  movable,  or  are  attached  to  the  wall  of  the 
furnace.  In  many  cases  they  are  water-cooled  and  sometimes  a 
lateral  holder  serves  as  a  gland  to  make  a  gas-tight  joint  between 
the  electrode  and  the  wall  or  roof  of  the  furnace. 


110 


THE  ELECTRIC  FURNACE 


Terminal  holders  are  attached  to  the  end  of  an  electrode  and 
are  usually  movable,  serving  to  advance  the  electrode  as  it  wears 
away.  The  typical  holder  of  this  class  is  used  in  pit  furnaces  for 
the  production  of  calcium  carbide,  ferro-silicon,  etc.  The  holder  is 
supported  by  a  chain,  and  lowered  as  far  as  practicable,  as  the 
electrode  burns  away.  The  holder  is  usually  water-cooled  and  is 
clamped  to  the  electrode  with  the  aid  of  bolts  or  wedges,  or  the  elec- 


CZJ 


FIG.  40. — Water-cooled  holder  for  square  electrodes. 

trode  end  may  be  threaded,  and  screwed  into  a  socket  in  the  holder. 
The  holder  is  usually  of  a  considerable  length  so  that  when  the 
lower  end  is  in  the  furnace  the  upper  end,  to  which  the  cables  are 
attached,  will  be  sufficiently  removed  from  the  heat  of  the  furnace. 
For  use  with  an  inclined  or  horizontal  electrode  the  holder  is  pro- 
vided with  guides  to  support  and  direct  the  electrode.  The  electric 
current  enters  the  electrode  through  the  holder  and  passes  along  the 


CONSTRUCTION  AND  DESIGN 


111 


whole  length  of  the  electrode  before  entering   the   charge;   thus 
causing  a  waste  of  electrical  energy. 

Fig.  39  shows  a  water-cooled  electrode  holder  such  as  is  used  in 
some  calcium  carbide  furnaces.  A  B  is  an  iron  casting  with  threaded 
sockets  into  which  the  round  electrodes  are  screwed.  The  block 
A  B  is  supported  from  another  cross-piece  C  D  by  the  pipes  C  A 
and  D  B  which  convey  both  the  cooling  water  and  the  electric 


FIG.  41. — Air-cooled  holder  for  square  electrodes. 

current.  The  holder  is  supported  by  a  pulley  and  chain,  which 
are  insulated  from  the  holder,  and  electrical  connection  is  made  by 
flexible  uncovered  cables  as  shown.  In  the  carbide  furnace,  two 
such  electrode  holders  are  used  as  shown  in  the  end  view  and  in 
Fig.  121,  the  current  entering  by  one,  passing  through  the  molten 
carbide  in  the  furnace,  and  out  by  the  other.  The  two  holders 
are  supported  by  the  same  pulley,  so  that  one  regulating  apparatus 


112 


THE  ELECTRIC  FURNACE 


serves  for  both.     A  similar  holder  for  use  with  laboratory  furnaces 
is  shown  in  Fig.  75. 

Fig.  40  shows  a  holder  designed  for  use  with  a  rectangular  elec- 
trode. The  electrode  is  grooved  on  both  sides  to  allow  the  passage 
of  the  bolts  and  this  prevents  any  possibility  of  its  slipping  from  the 
holder.  The  contact  plates  must  be  arranged  so  that  they  can  be 
tightened  on  to  the  electrode.  This  part  of  the  holder,  A  B,  is  con- 
nected to  the  upper  part,  C  D,  by  pipes  carrying  cooling- water.  The 
supporting  pulley  and  the  electric  cables,  which  are  not  shown  in 


FIG.  42. — Air-cooled  holder  for  square  electrodes. 

the  figure,  are  attached  to  C  D,  and  are  thus  removed  from  the  heat 
of  the  furnace. 

Fig.  41  shows  the  holder  used  in  the  Heroult  iron-smelting  furnace 
at  Sault  Ste.  Marie.1  This  consists  of  an  iron  head,  A,  having 
wedge-shaped  jaws,  /,  /,  which  are  prevented  from  spreading  by 
two  bolts.  The  electrode  is  recessed  to  fit  the  jaws  and  is  driven 
downward  by  wedges,  thus  making  good  contact  with  the  jaws. 
The  upper  part,  B,  is  made  of  sheet  copper,  and  enables  the  electric 
cable,  and  the  pulley  and  chain  by  which  the  electrode  is  suspended, 
to  be  placed  so  far  above  the  furnace,  that  they  will  not  be  over- 

1  Dr.  Haanel,  Report  on  Experiments  at  Sault  Ste.  Marie,  1907,  Plate  VII. 


CONSTRUCTION  AND  DESIGN 


113 


heated,  while  the  lower  part,  A,  can  be  cooled  by  air  or  water  intro- 
duced from  above. 

Fig.  42  shows  a  holder  used  in  some  French  carbide  furnaces.1 
It  resembles  the  last-mentioned  holder  but  has  no  provision  for 
water-cooling,  and  the  cables  are  attached  immediately  above  the 
electrode.  The  holder  and  cables  would  be  likely  to  suffer  from 
over-heating. 


FIG.  43. — Simple  threaded  holder. 

The  holders  used  in  the  Domnarfvet  iron  smelting  furnace,  Fig.  90, 
are  similar  to  this,  but  guides  are  needed  in  that  case  to  support 
and  direct  the  electrodes  which  are  inclined  at  about  60°  to  the 
horizontal. 

A  simple  holder  for  round  electrodes  with  threaded  ends  is  shown 
in  Fig.  43.  The  thread  in  the  socket  may  be  slightly  tapered  so  as 


FIG.  44. — Electrode  holder  for  carborundum  furnace. 

to  ensure  a  good  contact.  As  additional  security  one  or  more  set 
screws  may  enter  through  the  side  of  the  holder.  Cooling  water 
could  be  introduced,  if  desired,  through  the  supporting  rod. 

A  holder  of  this  type  is  used  in  the  Stassano  steel  furnace,  Fig.  109. 
In  this  case  the  part  of  the  electrode  outside  the  furnace  is  protected 
from  oxidation  by  the  surrounding  water-jacket,  which  also  serves 

1  W.  Borchers,  Electric  Furnaces,  p.  191. 
8 


114 


THE  ELECTRIC  FURNACE 


to  prevent  over-heating  of  the  holder.  The  rod  of  the  holder  passes 
through  a  stuffing-box  to  render  the  furnace  gas-tight. 

The  holders  used  in  the  Acheson  carborundum  and  graphite 
furnaces,  Figs.  8  and  44,  are  terminal  holders,  but  in  this  case  the 
electrodes  do  not  wear  but  remain  of  a  constant  length,  and  the 
holders  are  stationary.  The  electrodes  in  these  furnaces  are  built 
up  of  a  number  of  graphite  rods  of  rectangular  section.  A  number 
of  copper  strips  are  laid  between  these  rods  and  the  whole  is  clamped 
together.  Electrical  contact  is  made  to  the  ends  of  the  copper 
strips. 

Lateral  electrode  holders  hold  the  electrode  at  any  point  along 
its  length,  and  the  electrode  can  be  advanced  through  the  holder  as 
it  becomes  shorter.  With  those  holders  the  electric  current  does 


FIG.  45. — Water-cooled  lateral  electrode  holder. 

not  travel  so  far  through  the  electrode,  thus  avoiding  waste  of 
power,  but  on  the  other  hand  means  must  be  provided  for  feeding 
the  electrode  through  the  holder.  It  may  be  worth  while  to  mention 
that  the  waste  of  power  with  terminal  electrode  holders  is  caused 
mostly  by  the  change  in  the  effective  length  of  the  electrode  as  it 
wears  away.  If  the  electrode  is  correctly  proportioned  when  new, 
it  will  become  too  short,  in  proportion  to  its  cross-section,  as  it  wears 
away;  while  if  correctly  proportioned  for  its  final  length,  it  will  be  too 
long,  in  proportion  to  its  cross-section,  when  new. 

The  lateral  electrode-holder  in  its  simplest  form  consists  of  a  clamp 
encircling  the  electrode  and  making  electrical  contact  with  it. 
Such  a  holder,  designed  for  round  electrodes,  is  shown  in  Fig.  45, 
and  consists  of  two  half-round  brass  castings  which  are  clamped 


CONSTRUCTION  AND  DESIGN 


115 


together  around  the  electrode  by  nuts  on  the  threaded  rods  that 
support  them.  The  nuts  also  serve  to  hold  the  lugs  of  the  cables 
and  to  adjust  the  height  of  the  holder.  There  is  a  hollow  beading, 
in  both  halves  of  the  holder,  through  which  cooling  water  is  circu- 
lated. Any  adjustment  of  the  electrode  is  made  by  loosening  the 
nuts  and  moving  the  electrode  through  the  holder.  This  holder  was 
designed  by  the  author  for  use  in  the  laboratory  with  electrodes  of 
2-in.  or  3-in.  diameter;  a  liner  being  used  with  the  smaller  sizes. 

A  similar  holder  without  water-cooling,  Fig.  46,  is  intended  for 
use  with  small  round  electrodes  in  the  Moissan  furnace,  and  serves 
in  this  case  to  feed  the  electrode  into  the  furnace.  The  holder  is, 
however,  essentially  lateral,  as  it  can  be  kept  near  to  the  wall  of  the 
furnace;  the  clamps  being  loosened  and  the  holder  withdrawn  at 


FIG.  46. — Uncooled  lateral  electrode  holder. 

intervals  when  the  electrode  has  become  too  short.  The  holder  is 
so  shaped  as  to  hold  round  electrodes  of  different  diameters. 

The  holder  of  the  Heroult  steel  furnace,  Fig.  47,  may  be  classed  as 
a  lateral  holder,  as  the  electrode  can  be  fed  through  the  holder; 
though  the  electrode  could  not  conveniently  be  advanced  through 
the  holder  when  in  use. 

The  electrodes  of  the  Heroult  steel  furnace1  are  supported  by 
arms  from  the  back  of  the  furnace,  instead  of  by  chains;  this  con- 
struction being  better  adapted  to  a  tilting  furnace.  The  electrode 
is  square  in  section,  and  is  surrounded  by  four  contact  pieces,  one 
for  each  side.  One  of  these  pieces  is  attached  to  the  arm  and  the 
other  three  are  tightened  against  the  electrode  by  a  steel  strap, 
which  encircles  them,  and  is  drawn  tight  by  a  screw  contained  within 
the  arm. 

xDr.  Haanel,  European  Report,  1904,  Figs.  3-7. 


116 


THE  ELECTRIC  FURNACE 


The  holder  is  shown  in  outline  in  Fig.  47.     A  A  is  the  arm  with 
tightening  screw,  S  and  nut  N,  to  which  is  attached  the  strap 


FIG.  47. — Electrode-holder  of  Heroult  steel  furnace. 

F,  which  draws  the  contact  pieces  B,  C  and  D  against  the  electrode 
E,  and  the  latter  against  the  arm  A.  A  cable,  not  shown  in  the 
sketch,  is  bolted  to  A,  B,  C,  and  D,  thus  distributing  the  current 


FIG.  48. — Stuffing-box  electrode  holder. 

to  the  contact  pieces.     A  shield  is  provided  to  protect  the  holder 
from  the  heat  and  smoke  of  the  furnace. 


CONSTRUCTION  AND  DESIGN  117 

The  author  has  developed,  for  use  in  laboratory  furnaces,  a  very 
convenient  type  of  holder  which  is  attached  to  the  wall  or  roof  of 
the  furnace  and  serves  as  a  stuffing-box  to  render  the  furnace  gas- 
tight;  preventing  any  escape  of  gases  around  the  electrode.  This 
is  shown  in  Fig.  48,  and  consists  of  a  water-cooled  brass  stuffing-box 
which  can  be  bolted  to  the  metal  casing  of  the  furnace;  suitable 
insulation  being  used  to  prevent  electrical  contact.  Metallic  pack- 
ing is  used,  consisting  of  a  flexible  braid  of  fine  copper  wire;  and 
when  reasonable  care  is  taken,  this  will  carry  very  large  currents 
without  becoming  over-heated.  Electrical  connection  is  made  to  the 
holder  by  the  thick-walled  copper  pipes  which  convey  the  cooling 
water. 

A  similar  holder,  intended  for  a  large  furnace,  has  been  described 
by  Dr.  Haanel,  see  Fig.  8 1,  in  which  blocks  of  graphite  are  used  for 
the  conducting  packing  in  the  stuffing-box. 

Round  electrodes  are  now  used  in  the  Heroult  steel  furnace,  see 
Fig.  94.  A  suitable  holder  consists  of  a  water-cooled  copper  collar, 
divided  at  one  point,  and  cut  partly  through  at  several  points  so 
that  it  can  be  tightened  or  loosened  by  two  lugs  and  a  right  and 
left  handed  screw. 

The  collar  is  supported  by  a  steel  arm  which  can  be  raised  or 
lowered  to  adjust  the  electrode  in  the  furnace.  The  collar  does  not, 
however,  make  electrical  contact  with  the  arm,  but  is  hung  from  it 
by  a  number  of  insulated  bolts;  electrical  connection  being  made  by 
flexible  cables  to  the  copper  collar  and  not  to  the  supporting  arm. 

When  the  electrode  has  become  too  short,  a  fresh  length  can  be 
screwed  on  above  the  old  one,  and  by  adjusting  the  screw  the 
lengthened  electrode  can  be  allowed  to  slide  down  through  the 
collar.1 

1  The  writer  is  indebted  for  particulars  of  this  holder  to  Mr.  W.  R.  Walker  of 
the  U.  S.  Steel  Corporation. 


CHAPTER  V 

THE  OPERATION  OF  ELECTRIC  FURNACES 
ELECTRICAL  SUPPLY 

Alternating  Current. — For  operating  electric  furnaces  alternating 
current  is  usually  employed,  on  account  of  the  greater  ease  with 
which  it  can  be  produced,  and  because  it  can  be  changed  into  cur- 
rent at  higher  and  lower  voltages  by  means  of  a  static  transformer 
—that  is,  without  the  need  of  some  rotating  machinery  such  as 
would  be  needed  for  changing  the  voltage  of  direct  current. 

In  general,  electric  power  for  smelting  or  other  purposes  is  gener- 
ated at  some  point  where  water-power  is  available.  The  power  is 
conveyed  by  an  alternating  electric  current  of  high  voltage,  to  the 
point  at  which  the  electric  smelting  is  to  be  done.  At  this  point  it 
is  changed  by  means  of  a  static  transformer  into  a  much  larger  cur- 
rent, at  the  low  voltage  which  is  suitable  for  electric  smelting.  In 
order  to  regulate  the  voltage  of  the  current  flowing  to  the  furnace 
the  secondary  winding  of  the  transformer  is  often  divided  into  two 
parts,  and  by  coupling  these  in  series  or  parallel,  the  current  can  be 
obtained  at  either  of  two  voltages,  one  of  which  is  twice  the  other. 
Smaller  variations  in  the  voltage  of  the  secondary  current  are  ob- 
tained by  means  of  taps  on  the  primary  winding,  which  render  it 
possible  to  alter  the  number  of  effective  turns  of  the  primary  winding. 
By  cutting  out  one  section  after  another,  of  this  winding,  the  ratio 
between  the  turns  in  the  primary  and  secondary  is  decreased,  and 
therefore  the  voltage  in  the  secondary  is  correspondingly  raised. 

An  example  of  such  a  transformer,  with  regulation  of  voltage,  is 
shown  in  Fig.  49,  which  is  a  diagrammatic  view  of  one  of  the  i,6oo-kw. 
transformers  in  the  Carborundum  Works  at  Niagara  Falls.1  The 
transformer  is  supplied  with  primary  current  at  2,200  volts,  while 
the  secondary  current,  which  passes  through  the  furnace,  can  have 
any  voltage  between  40  and  200.  In  the  diagram,  the  secondary 
windings  55  are  divided  into  halves,  and  these  by  means  of  a  switch 
W  can  be  coupled  in  series  or  parallel.  The  primary  winding  PP 

1  E.  F.  Gehrkens,  "Voltage  Control  of  Transformers  for  Electrical  Furnaces." 
Met.  and  Chem.  Engng.,  viii,  1910,  p.  373. 

118 


THE  OPERATION  OF  ELECTRIC  FURNACES        119 

has  13  taps  on  it,  marked  i  to  13.  One  cable,  14,  from  the  supply 
Z,,  passes  to  one  end  of  the  winding,  while  the  other  cable  makes 
contact  at  any  one  of  the  13  taps.  In  changing  the  connection  from 
one  tap  to  the  next,  it  is  necessary  to  make  contact  with  the  second 
tap  before  breaking  contact  with  the  first.  On  the  other  hand,  it 
would  not  do  to  make  direct  contact  with  two  taps  at  the  same  time, 
as  this  would  short-circuit  a  section  of  the  winding.  These  difficul- 
ties are  avoided  by  means  of  a  compensating  device,  shown  diagram- 
matically  at  AB,  which  can  be  connected  in  turn,  to  any  two  adjacent 
taps.  The  high-tension  terminal  makes  contact  through  a  number 
of  fingers  T,  to  points  on  the  resistance  wire  AB.  Supposing,  then, 
that  T  is  opposite  At  contact  is  being  made  directly  to  tap  6.  By 


FIG.  49. — Transformer  with  voltage  regulation. 

moving  T  .gradually  from  A  to  B,  the  connection  is  changed  from 
tap  6  to  7  without  any  sudden  make  or  break.  In  order  to  change 
the  connection  from  7  to  8,  the  apparatus  AB  is  reversed,  so  that  the 
contact  which  was  on  6  comes  to  8,  and  then,  by  moving  T  gradually 
back  from  B  to  A,  the  contact  is  transferred  from  7  to  8. 

In  order  to  avoid  the  production  of  dangerously  high  voltages 
in  the  unused  portions  of  the  primary  winding,  switches  are  placed 
in  this,  at  the  points  G  and  H,  which  can  be  opened  when  the  coils 
to  the  left  of  them  are  out  of  use.  When  the  secondary  windings 
are  coupled  in  parallel  and  the  whole  of  the  primary  winding  is  in 
use,  the  voltage  supplied  to  the  furnace  will  be  40,  and  under  these 
conditions  a  current  of  40,000  amperes  can  be  supplied  to  the  fur- 
nace. By  moving  the  contact  piece  gradually  from  i  to  13,  the 
voltage  at  the  furnace  will  rise  from  40  to  100.  Coupling  the 


120 


THE  ELECTRIC  FURNACE 


secondary  coils  in  series  would  double  this  voltage,  and  therefore 
in  order  to  raise  the  voltage  gradually  it  is  necessary,  at  the  same 
time,  to  change  the  contact  in  the  primary  winding  back  to  some 
point,  which  shall  give  100  volts  at  the  furnace.  By  again  advanc- 
ing the  contact  up  to  tap  13,  the  voltage  at  the  furnace  will  have 
risen  to  200;  under  which  conditions  the  furnace  will  only  draw 
8,000  amperes.  The  apparatus,  described  here  in  outline,  is  con- 
structed so  as  to  work  almost  automatically  by  means  of  an  electric 
motor. 

Direct  Current. — In  electric-furnace  operation  direct  current  is 
only  employed  when  it  is  needed  for  electrolysis.  It  is  obtained 
by  means  of  a  direct-current  generator  driven  by  a  prime  mover 
or  by  an  electric  motor  operated  by  the  alternating- current  supply. 
The  alternating-current  motor  and  direct-current  generator  may 
be  replaced  by  a  motor-generator  which  is  supplied  with  alternat- 
ing current  and  delivers  direct  current.  In  consequence  of  the 
very  low  voltage  of  most  electrolytic  furnaces,  a  number  of  these 
are  usually  connected  in  series,  thus  avoiding  the  need  of  a  very 
low-voltage  generator. 

Polyphase  Currents. — Electric  power,  in  large  quantities,  is 
usually  produced  in  the  form  of  three-phase  current  and  it  is  there- 


FIG.  50. — Three-phase  furnaces. 

fore  desirable  to  employ  this  instead  of  single-phase  current  in 
electric  smelting.  In  some  furnaces  three-phase  current  can  be 
used  to  advantage,  and  is  preferable  to  single-phase  current.  Such 
furnaces  are  provided  with  three  or  six  electrodes,  connected  to  the 
three-phase  supply  as  shown  in  Fig.  50.  Other  furnaces  can  only 
be  operated  by  single-phase  current,  and  in  such  cases  three  fur- 
naces should  be  grouped  as  one  unit,  each  using  one  phase  of  the 
three-phase  supply. 


THE  OPERATION  OF  ELECTRIC  FURNACES        121 

In  the  latter  case  there  are  two  ways  in  which  the  furnaces  may 
be  connected.  In  one  of  these — the  delta  connection — see  Fig. 
51,  each  furnace,  F,  is  connected  between  two  of  the  cables  i,  2,  3; 
each  cable  being  connected  to  one  electrode  of  two  furnaces.  The 
other  connection— the  F— is  shown  in  Fig.  52.  Here  one  terminal 
of  each  furnace  is  connected  to  one  cable  and  the  other  terminal  of 


FIG.  51. — Delta  connection  of  furnaces. 

each  furnace  is  connected  to  a  common  point.  In  this  connection 
the  voltage  supplied  to  each  furnace  is  only  58  per  cent,  of  the 
voltage  obtained  by  the  first  connection,  but  the  current  is  propor- 
tionally larger,  producing  an  equal  amount  of  power.  The  arrange- 
ment of  the  furnaces  in  Figs.  51  and  52  is  made  so  as  to  show  the 
current  connections  more  clearly,  the  direction  of  each  furnace 
showing,  in  angular  measure,  the  relative  phase  displacement  of  the 


FIG.  52. — Y  connection  of  furnaces. 

voltage  supplied  to  it.  The  furnaces  would  not  actually  be  arranged 
in  this  manner.  In  these  figures  the  furnaces  have  been  shown  as 
connected  directly  to  the  conductors  bringing  the  current,  but  it  is 
usually  necessary  to  employ  transformers  to  lower  the  voltage. 
One  transformer  may  be  used  for  each  furnace,  or  the  three  trans- 
formers may  be  combined  to  form  a  three-phase  transformer  taking 


122 


THE  ELECTRIC  FURNACE 


in  three-phase  current  at  a  high  voltage  and  giving  out  low- voltage 
three-phase  current  which  can  be  supplied  to  the  furnaces  as  shown 
in  the  figures. 

Three-phase  current  has  been  used  in  the  Heroult  steel  furnace, 
Fig.  94,  using  three  electrodes,  and  in  the  Domnarfvet  iron  smelting 
furnace,  Fig.  90,  using  three  or  six  electrodes.  In  the  Gronwall 
steel  furnace,  Fig.  100,  an  electrode  is  provided  in  the  bottom  of 
the  furnace  in  order  to  produce  more  heat  in  the  molten  steel.  If 
three  movable  electrodes  were  used  and  the  bottom  electrode  were 
made  the  neutral  point,  the  current  would  circulate  between  the 
movable  electrodes  and  hardly  any  would  pass  down  to  the  bottom 
electrode,  but  by  using  only  two  movable  electrodes  the  current 
can  be  made  to  flow  through  the  bottom  electrode.  For  this  pur- 
pose three-phase  current  should  not  be  used  because,  as  the 


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FIG.  53. — Scott  connection  of  transformers. 

resistance  between  the  two  movable  electrodes  is  decidedly  greater 
than  between  either  of  these  and  the  bottom  electrode,  the  phases 
would  be  unbalanced.  Two-phase  current  is  therefore  employed; 
the  bottom  electrode  serving  as  a  common  return.  Two-phase 
current  has  also  been  used  in  the  iron  smelting  furnace  at  Troll- 
hattan,  using  four  movable  electrodes. 

Three-phase  current  is  more  usually  available  than  two-phase, 
and  when  it  is  desired  to  convert  three-phase  to  two-phase  current — • 
or  the  reverse — -the  conversion  can  easily  be  effected,  at  the  same 
time  as  the  necessary  change  from  high-tension  to  low- tension,  by 
means  of  transformers  arranged  with  the  Scott  connection.  This  is 
shown  in  Fig.  53,  in  which  high-tension  three-phase  current  is  being 
changed  into  low- tension  two-phase  current  for  use  in  the  furnace. 
In  the  figure  ab,  cd  are  the  high-tension  windings  and  AB,  CD  the 


THE  OPERATION  OF  ELECTRIC  FURNACES        123 

low-tension  windings  of  two  equal  transformers  giving  the  desired 
voltage  ratio  between  the  three-phase  supply,  i,  2,  3,  and  the  two- 
phase  current  to  be  produced.  The  transformer  cd  has  a  tap,  /,  at 
the  middle  of  the  high-tension  windings,  and  the  transformer  ab 
has  a  tap,  e,  so  that  eb  is  87  per  cent,  of  ab.  When  the  connections 
are  made  as  indicated,  the  low- tension  currents  obtained  from  the 
windings  AB  and  CD  will  be  of  equal  voltage  and  will  have  a  phase 
difference  of  one-quarter  period;  that  is  they  will  constitute  a  two- 
phase  current.  As  the  section  ae  of  the  high-tension  winding  is  not 
used  in  this  arrangement,  it  can  be  omitted,  but  it  is  sometimes 
convenient  to  use  a  standard  transformer  with  an  87  per  cent,  tap, 
as  shown  in  the  figure. 

Electric  Power. — In  the  case  of  a  direct  current  or  an  alternating 
current  supplying  a  non-inductive  furnace,  the  product  of  the  am- 
peres flowing  through  the  furnace  and  the  voltage  measured  between 
its  terminals  gives  the  power  in  watts,  that  is  the  rate  at  which 
energy  is  being  supplied  to  the  furnace.  Most  furnaces  are  some- 
what inductive,  partly  on  account  of  the  iron  used  in  their  con- 
struction, and  when  these  are  operated  by  an  alternating  current 
the  inductance  of  the  furnace  causes  the  current  to  lag  behind  the 
applied  voltage.  Under  these  conditions  the  volt-amperes,  that  is, 
the  product  of  the  volts  and  the  amperes,  is  larger  than  the  true 
watts  expended  in  the  furnace.  The  ratio  of  the  watts  to  the  volt- 
amperes  supplied  to  the  furnace,  is  its  "power-factor."  The  power- 
factor  is  usually  high  in  electric  furnaces,  often  over  90  per  cent. ;  but 
in  induction  furnaces,  such  as  the  Kjellin  furnace,  it  is  frequently 
much  lower,  sometimes  as  low  as  60  per  cent. 

A  low  power-factor  is  very  undesirable  because,  for  the  same 
power,  the  volt-amperes  and  consequently  the  cost  of  the  electric 
generating  plant  will  be  higher  than  with  a  high  power-factor. 

To  keep  the  power-factor  as  high  as  possible  one  should  avoid 
carrying  heavy  currents  through  closed  loops  of  iron,  and  the  out- 
ward and  returning  conductors  should  be  kept  as  close  together  as 
possible.  For  this  reason  the  bus-bars  carrying  the  current  to  and 
from  a  furnace  are  often  interlaced  as  shown  in  the  illustration  of  the 
Keller  furnace,  Fig.  96.  The  power-factor  of  a  furnace  is  not  a 
function  of  its  construction  merely  but  depends  also  on  the  frequency 
of  the  current  supplied  to  it;  low-frequency  current  giving  a  higher 
factor  than  high-frequency  current.  On  this  account  2 5- cycle 
current  is  often  used  in  electric  smelting  in  preference  to  6o-cycle 
current. 


124 


THE  ELECTRIC  FURNACE 


Electric  Measurements. — In  electric  smelting,  the  voltmeter  and 
voltage  coil  of  the  wattmeter  can  be  connected  directly  to  the  furnace 
terminals,  or  the  bus-bars  supplying  these,  as  electric  furnaces  are 
almost  always  operated  at  moderate  voltages;  but  the  currents 
supplied  to  electric  furnaces  are  in  general  very  large  and  only  a 
small  fraction  of  the  whole  current  flows  through  the  ammeter  and 
current  coil  of  the  wattmeter  for  the  purpose  of  measurement.  In 
the  case  of  direct  currents  this  is  effected  by  means  of  a  shunt 


FIG.  54. — Shunt  and  ammeter. 

(marked  S  in  Fig.  54) ;  this  has  a  low  resistance  (between  the  points 
a  and  b)  which  is  an  exact  fraction  of  the  resistance  of  the  ammeter 
A  and  connecting  wires.  The  indication  of  the  ammeter  must  there- 
fore be  multiplied  by  a  factor  to  give  the  value  of  the  whole  current. 
In  the  measurement  of  alternating  current  a  current- transformer  is 
used.  This  is  a  small  transformer  Fig.  55,  having  the  low- tension 
winding  S  in  the  main  circuit  and  the  high-tension  winding  s  in 
circuit  with  the  ammeter  and  the  wattmeter.  The  current  in  this 
circuit  is  a  definite  fraction  of  the  current  in  the  main  circuit  and 


FIG.  55. — Current  transformer. 

the  readings  of  both  the  ammeter  and  the  wattmeter  must  be  multi- 
plied by  a  constant  to  obtain  the  amperes  and  watts  supplied  to 
the  furnace  F.  In  most  cases  the  low- tension  winding  S  is  not  a 
part  of  the  current  transformer  but  is  merely  a  piece  of  the  cable, 
or  conductor,  carrying  the  current  to  the  furnace.  The  current 
transformer  then  consists  of  the  high-tension  winding  5  and  the  iron 
core,  which  passes  through  the  winding  s  and  can  be  looped  around 
the  main  conductor. 


THE  OPERATION  OF  ELECTRIC  FURNACES        125 

When  using  two-phase  current,  an  ammeter,  voltmeter  and  watt- 
meter, would  be  needed  for  each  phase,  and  the  total  power  would 
be  the  sum  of  the  two  wattmeter  readings.  Fig.  56  shows  how  these 
would  be  arranged  in  the  case  of  a  two-phase  furnace  having  two 


FIG.  56. — Two-phase  furnace  measurements. 

supply  cables,  i  and  2,  and  a  common  return  R.  The  ammeters  and 
wattmeters  have,  for  simplicity  in  the  figure,  been  shown  in  the  main 
circuit. 

When  three-phase  current  is  supplied  to  three  furnaces,  as  in 


FIG.  57. — Three-phase  furnace  measurements. 

Figs.  51  and  52,  each  furnace  has  single- phase  current  and  will  be 
provided  with  separate  meters  as  in  Fig.  55;  but  when  a  single 
furnace  is  operated  by  three-phase  current,  two  wattmeters,  con- 
nected as  in  Fig.  57,  will  suffice  to  measure  the  total  power.  In  this 


126  THE  ELECTRIC  FURNACE 

case  conductor  3  may  be  regarded  as  the  return  from  i  and  2  and  the 
sum  of  the  two  readings  of  Wi  and  Wz  gives  the  whole  power  of 
the  furnace.  An  ammeter  should  be  provided  for  each  of  the  three 
electrodes  in  order  that  the  currents  drawn  by  each  may  be  kept 
equal. 

RATE  OF  HEAT  PRODUCTION  IN  ELECTRIC  FURNACES 

As  already  mentioned,  the  rate  at  which  heat  is  produced  in  an 
electric  furnace  may  be  measured  by  the  number  of  watts  of  electrical 
power  supplied  to  the  furnace,  allowance  being  made  when  necessary 
for  any  electrolysis  that  takes  place.  A  certain  rate  of  heating 
is  necessary  for  the  attainment  of  a  definite  temperature;  this 
rate  depending  on  the  thickness  and  heat-retaining  qualities  of 
the  furnace  walls,  upon  the  size  of  the  furnace,  and  upon  any  cooling 
influence,  such  as  the  introduction  of  fresh  ore  to  the  furnace.  The 
greater  the  rate  of  heat  production  in  an  electric  furnace,  the 
greater  in  general  will  be  its  efficiency.1  A  few  examples  will  now 
be  given  of  the  rate  of  heat  production  in  typical  electric  furnaces, 
the  rate  being  given  in  watts  per  cubic  inch,  or  in  kilowatts  per  cubic 
foot.2 

In  Moissan's  small  furnace,  Fig.  6,  which  was  composed  of  blocks 
of  quicklime,  he  employed  35  to  40  amperes  of  direct  current  at 
55  volts,  or  1,900  to  2,200  watts.3  The  interior  cavity  of  the  furnace 
was  about  1.75  in.  in  diameter,  and  about  1.7  in.  in  height,  corre- 
sponding to  a  volume  of  4.1  cu.  in.  The  watts  per  cubic  inch  will, 
.therefore,  be  470  to  540,  or  say  500  as  a  round  figure.  Some  allow- 
ance should  be  made  for  the  heat  produced  in  the  electrodes  them- 
selves, and  this  would  leave  perhaps  400  or  450  watts  per  cubic 
inch  for  the  interior  of  the  furnace.  This  figure,  as  will  be  seen 
directly,  is  about  100  times  as  great  as  the  usual  rate  of  heating  in 
a  fair-sized  electric  furnace,  as  used  for  steel-making,  for  instance. 

Moissan's  electric  tube  furnace,  contained  a  carbon  tube  in  which 
the  material  to  be  heated  was  placed,  and  the  furnace  itself  was 
composed  of  limestone,  and  was  lined  with  alternate  layers  of 
carbon  and  magnesia.  In  this  furnace  he  employed  300  amperes 
at  70  volts  (=21,000  watts),  or  1,000  amperes  at  60  volts  (  =  60,000 

1  Attention  has  been  called  to  this  feature  of  electrical  heating  by  Dr.  Carl 
Hering,    "Advantages   of    Small   High-speed   Electric   Furnaces."     Met.    and 
Chera.  Eng.,  xi,  1913,  p.  183. 

2  i  Watt  per  cubic  inch  =  1.7 28  kw.  per  cubic  foot. 

3  H.  Moissan,  The  Electric  Furnace,  p.  5 


THE  OPERATION  OF  ELECTRIC  FURNACES        127 

watts).1  The  dimensions  of  the  interior  of  the  furnace,  assuming 
that  his  perspective  drawings  are  to  scale,  would  be  4.4  in.  long, 
3.2  in.  wide,  and  4  in.  high;  corresponding  to  a  volume  of  56  cu.  in. 
The  watts  per  cubic  inch  would  vary  from  380  to  1,100.  A  deduction 
for  the  heat  produced  in  the  electrodes  themselves  would  reduce 
these  figures  by  10  per  cent,  or  20  per  cent. 

An  even  more  intense  rate  of  heating  is  mentioned,  in  which  he 
employs  1,200  to  2,000  amperes  at  100  volts  in  an  unlined  limestone 
furnace.2  The  internal  diameter  is  stated  to  be  4  in.,  and  assuming 
the  height  to  be  the  same,  the  volume  of  the  cavity  would  be  50 
cu.  in.  The  watts  supplied  would  be  120,000  to  200,000,  or  2,400 
to  4,000  watts  per  cubic  inch.  The  operation  of  this  furnace  is 
only  of  short  duration,  the  lime,  produced  by  heating  the  interior  of 
the  limestone  blocks,  soon  melting,  and  running  like  water,  while 
vaporized  lime  roars  out  around  the  electrodes,  and  the  furnace  is 
soon  destroyed.  The  temperature  produced  was  limited  by  the 
rapid  melting  and  vaporizing  of  the  lime,  but  by  supplying  the  heat 
at  such  an  enormous  rate,  the  greater  part  of  the  cavity  might  well 
be  considerably  hotter  than  the  boiling  temperature  of  melted  lime. 

The  small  furnace,  first  mentioned,  could  be  used  for  longer 
periods,  as  the  rate  of  heat  production  was  so  much  less,  and  the 
furnace  was  therefore  less  rapidly  destroyed;  while  the  tube  furnace, 
lined  with  carbon  and  magnesia,  could  be  run  continuously. 

The  Stassano  steel  furnace,  Fig.  109,  resembles  the  Moissan 
furnace,  as  the  ore  to  be  smelted  is  heated  by  radiation  from  an  arc. 
The  furnace  described  by  Dr.  Goldschmidt  in  1903, 3  which  is  some- 
what larger  than  the  one  figured  in  Dr.  Haanel's  Report,4  took  an 
alternating  current  of  2,000  amperes  at  170  volts,  and  used  about 
450  h.p.  The  horse-power  corresponds  to  336  kw.,  but  part  of  this 
would  be  wasted  outside  the  furnace.  The  volt-amperes  are 
340,000,  and  assuming  a  power- factor  of  0.75,  this  would  give  255  kw. 
consumed  in  the  furnace.  The  interior  of  the  furnace  was  about 
4o-in.  cube,  or  64,000  cu.  in.,  giving  4  watts  per  cubic  inch,  or  6.9 
kw.  per  cubic  foot.  The  difference  between  this  rate  of  heating 
and  that  employed  by  Moissan,  depends  in  part  upon  the  lower 
temperature  required,  in  part  upon  the  great  loss  of  heat  produced 
by  the  vaporizing  of  the  materials  of  the  Moissan  furnace,  and  in 

1  H.  Moissan,  The  Electric  Furnace,  p.  17. 

2  H.  Moissan,  The  Electric  Furnace,  p.  14. 

3  Electrochemical  Industry,  vol.  i,  p.  247. 

4  European  Commission  Report,  p.  n  and  Figs.  9  and  10. 


128  THE  ELECTRIC  FURNACE 

part  upon  the  larger  size  and  better  heat-retaining  construction  of 
the  Stassano  furnace. 

The  Heroult  steel  furnace  at  La  Praz,  Fig.  93,  figured  by  Dr. 
Haanel,1  is  about  7  ft.  long,  4  ft.  wide,  and  2  ft.  high  inside,  giving 
a  volume  of  56  cu.  ft.  The  power  employed  was  353  kw.,2  or  6.3 
kw.  per  cubic  foot,  which  agrees  well  with  the  Stassano  furnace. 

The  furnace  employed  at  Sault  Ste.  Marie,  for  smelting  Canadian 
iron-ores,  Fig.  78,  had  an  interior  volume  of  18.4  cu.  ft.,  and  con- 
sumed about  1 66  kw.  of  electrical  power,  or  9  kw.  per  cubic  foot.3 
This  is  only  a  little  larger  than  the  figures  for  the  Heroult  steel  fur- 
nace:— -6.3  kw.  per  cubic  foot,  and  the  Stassano  furnace, — 6.9  kw. 
per  cubic  foot;  but  the  meaning  of  the  figure  is  not  quite  the  same. 
The  whole  interior  of  the  Stassano  and  Heroult  steel  furnaces  is 
heated  to  about  the  melting  temperature  of  the  steel,  and  the  rate 
of  heat  production  for  each  cubic  foot  of  the  furnace  is  of  the  first 
importance  in  determining  the  temperature  to  which  the  furnace 
can  be  heated.  In  the  Heroult  ore  smelting  furnace,  however,  the 
temperature  is  far  from  uniform  throughout  the  interior,  only  the 
lower  part  being  heated  to  a  smelting  temperature;  and  the  volume 
of  the  upper  part  of  the  furnace,  where  the  ore  is  gradually  heated 
during  its  descent  to  the  smelting  zone,  could  be  very  much  greater 
without  the  change  having  any  material  effect  upon  the  temperature 
in  the  smelting  zone  of  the  furnace.  In  such  a  furnace  it  is  con- 
sequently of  little  importance  to  consider  the  total  volume  in  rela- 
tion to  the  electrical  power,  a  more  significant  figure  being  obtained 
by  dividing  the  kilowatts  by  the  volume  in  cubic  feet  of  the  fusion  or 
smelting  zone  of  the  furnace.  This  zone  is  necessarily  difficult  to 
define,  but  assuming  that  the  electrode,  C,  in  Fig.  78,  is  in  its  nor- 
mal working  position,  the  smelting  zone  would  occupy  about  7 
cu.  ft.,  making  the  electrical  power  24  kw.  per  cubic  foot  of  the 
zone. 

The  rate  of  heating,  in  the  smelting  zone  of  this  furnace,  is  very 
much  greater  than  in  a  steel  furnace,  and  this  is  explained  by  the 
constant  supply  of  cooler  material  which  absorbs  most  of  the  heat. 
The  efficiency  will  tend  to  increase  as  the  furnace  is  driven  faster; 
but  with  the  more  rapid  smelting,  the  zone  of  fusion  will  become 
enlarged;  thus  corroding  the  walls  of  the  furnace.  There  is  con- 

1  European  Commission  Report,  p.  5  and  Figs.  3  and  4. 

2  European  Commission  Report,  p.  53. 

3  Dr.  Haanel,  Report  on  Experiments  at  Sault  Ste.  Marie,  1907,  p.  46,  and  plate 
vii.     (Also  in  the  "Canadian  Engineer,"  vol.  xiii,  pp.  221  and  254.) 


THE  OPERATION  OF  ELECTRIC  FURNACES       129 

sequently  a  limit  beyond  which  it  is  not  desirable  to  increase  the 
rate  of  heating  in  electric  furnaces. 

The  Keller  furnace,  Fig.  79,  consists  of  two  smelting  shafts,  with 
a  common  reservoir  for  the  molten  products.  Taking  the  dimen- 
sions from  Figs,  n  and  12  of  Dr.  HaanePs  European  report,  the 
smelting  zone,  AB,  of  each  shaft,  omitting  the  connecting  passage, 
CC1,  which  acts  as  a  reservoir  for  the  fused  iron  and  slag,  will  occupy 
about  19  cu.  ft.  The  power  used,  in  the  first  run  of  furnaces,  Nos. 
ii  and  12,  was  613  kw.,1  or  306  kw.  for  each  of  the  two  shafts.  This 
is  equal  to  16  kw.  for  each  cubic  foot  of  the  fusion  zone.  If  the 
whole  volume  of  the  shaft  were  considered,  the  power  would  corre- 
spond to  5  kw.  per  cubic  foot,  or  to  6  kw.  per  cubic  foot  of  the  shaft 
up  to  the  level,  FG,  at  which  the  gases  escape  from  the  furnace. 

These  figures  are  less  than  were  obtained  from  the  Heroult  fur- 
nace; the  difference  being  mainly  due  to  the  larger  size  of  the  Keller 
furnace,  in  which  the  smelting  zone  was  three  times  as  large  as  in 
the  Heroult  furnace.  The  larger  size  of  the  Keller  furnace  occa- 
sioned a  smaller  loss  by  radiation  and  conduction  per  cubic  foot,  and 
a  correspondingly  smaller  rate  of  heat  production  was  required.  In 
this  connection  it  should  be  mentioned,  that  the  rate  of  smelting, 
per  cubic  foot  of  smelting  zone,  in  the  Keller  furnace,  was  less  than 
one- third  of  the  rate  of  smelting  in  the  Heroult  furnace,  while  the 
consumption  of  electrical  energy  per  ton  of  pig-iron  was  twice  as 
great  in  the  Keller  furnace  as  in  the  Heroult  furnace.  This  seems  to 
indicate  that  the  supply  of  power  in  the  Keller  furnace  was  not  quite 
sufficient;  but  as  this  furnace  was  working  badly  as  a  result  of  a 
shut-down,  it  is  unsafe  to  draw  deductions  from  its  rate  of  smelting. 

Better  results  were  obtained  in  the  second  run,  with  the  Keller 
furnaces  Nos.  i  and  2.2  These  were  stated  to  be  identical  with 
Nos.  ii  and  12,  with  the  exception  of  the  connecting  channel, 
which  was  absent  in  Nos.  i  and  2.  Assuming  the  smelting  zone 
to  be  of  the  same  size,  the  rate  of  heat  production  would  be  only 
6  kw.  per  cubic  foot  of  this  part  of  the  furnace.  The  energy  con- 
sumption per  ton  of  pig-iron,  in  these  furnaces,  was  a  little  less  than 
in  the  Heroult  furnace. 

In  the  Kjellin  steel  furnace,  Fig.  102,  no  electrodes  are  em- 
ployed: the  steel  is  contained  in  a  ring-shaped  trough,  and  is  melted 
by  an  electric  current  which  is  induced  in  the  steel  just  as  in  the 
secondary  windings  of  a  transformer. 

1  Dr.  Haanel's  European  Report,  1904,  p.  40. 

2  Dr.  Haanel's  European  Report,  1904,  p.  44. 

9 


130  THE  ELECTRIC  FURNACE 

The  furnace  shown  in  Dr.  Haanel's  report,1  has  a  trough  of  13 
cu.  ft.  capacity.  The  power  delivered  to  the  primary  of  the  trans- 
former was  150  kw.  Assuming  the  transformer  losses  to  be  10  per 
cent,  of  this,  135  kw.  would  be  supplied  to  the  molten  steel,  or  10 
kw.  per  cubic  foot  of  the  furnace. 

This  figure  is  larger  than  in  the  Heroult  steel  furnace,  and  the 
difference  may  be  partly  accounted  for  by  the  larger  amount  of 
waste  space  in  the  latter  furnace.  The  efficiency  of  the  Kjellin 
furnace  is  low,  on  account  of  the  small  cross-section  (6  in.  by  18  in.) 
of  the  trough  containing  the  molten  steel;  and  a  somewhat  small 
cross-section  appears  to  be  necessary  in  this  type  of  furnace. 

The  Gin  steel  furnace,  Fig.  108,  resembles  the  Kjellin  furnace  in 
consisting  of  a  long  trough  or  canal,  of  small  cross- section,  contain- 
ing the  molten  steel;  but  the  electric  current  is  introduced  at  the 
ends  of  the  trough  through  water-cooled  steel  terminals.  In  order 
to  reduce  the  loss  of  heat,  the  canal  is  folded  upon  itself  like  the  fila- 
ment of  an  incandescent  lamp. 

Mr.  Gin  calculates  the  dimensions  for  a  number  of  furnaces  in  a 
paper  that  has  been  printed  in  Dr.  Haanel's  report.2  For  a  700- 
kw.  furnace,  the  volume  of  the  steel  in  the  trough  would  be  19.5 
cu.  ft.,  and  assuming  the  trough  to  be  half  filled  by  the  molten  steel, 
its  capacity  would  be  39  cu.  ft.,  corresponding  to  18  kw.  per  cubic 
foot  of  the  trough.  The  trough  would  be  nearly  30  ft.  long,  9  3/4 
in.  wide,  19  1/2  in.  deep,  and  half  full  of  molten  steel. 

In  the  Acheson  furnaces,  Figs.  8  and  115-117,  the  heat  is  produced 
by  the  passage  of  an  electric  current  through  a  central  core  or  through 
the  charge  itself.  The  charge  does  not  melt,  and  remains  nearly 
in  the  same  position  until  the  end  of  the  operation.  Very  little 
data  is  available  with  regard  to  the  actual  working  of  these  furnaces. 
The  following  examples  may  be  given: — • 

In  a  patent  by  Dr.  Acheson3  for  a  method  of  making  carbon 
articles  of  a  high  density  and  conductivity  by  heating  them  in  an 
electric  furnace  without  reaching  the  point  of  graphitization,  the 
furnace  is  stated  to  be  30  ft.  long,  30  in.  wide,  and  10  in.  deep  inside. 
The  power  used  was  about  750  kilo- volt-amperes.  Assuming  the 
power-factor  to  be  0.9  this  would  correspond  to  10  or  n  kw.  per 
cubic  foot  of  the  furnace. 

1  Dr.  Haanel's  European  Report,  1904,  p.  2  and  Fig.  i. 

2  Dr.  Haanel's  European  Report,  1904,  p.  173. 

3  E.  G.  Acheson,  U.  S.  patent  749,418,  see  Electrochemical  Industry,  vol.  ii, 
p.  108. 


THE  OPERATION  OF  ELECTRIC  FURNACES        131 

The  Acheson  graphite  and  electrode  furnaces  are  described  by 
F.  A.  J.  FitzGerald1  who  takes  the  data  in  part  from  Acheson's 
patents.  The  graphite  furnace  is  said  to  be  30  ft.  long,  14  in.  wide, 
and  1 8  in.  deep,  thus  having  a  volume  of  52  1/2  cu.  ft.2  The  current 
at  the  beginning  of  the  run  is  stated  to  be  3000  amperes  at  200  volts, 
and  FitzGerald  assumes  that  when  the  furnace  has  become  heated  it 
absorbs  750  kw.  The  latter  figure  would  correspond  to  14  kw.  per 
cubic  foot.  In  the  electrode  furnace,  the  length  between  terminals 
is  30  ft.,  and  the  cross-section  of  the  piles  of  electrodes  under  treat- 
ment is  24  in.  by  17  in.  Allowing  a  few  inches  of  granular  carbon 
around  the  electrodes,  the  volume  of  the  furnace  would  be  150  cu.  ft.; 
700  kw.  were  employed,  corresponding  to  less  than  5  kw.  per  cubic 
foot.  If  no  allowance  were  made  for  the  granular  carbon,  the  rate 
of  heating  would  be  8  kw.  per  cubic  foot  of  the  charge. 

A  drawing  of  the  carborundum  furnace3  shows  it  to  be  16.5 
ft.  long,  6  ft.  wide  and  5.5  ft.  high  inside.  The  power  used  is  750 
kw.,  which  is  only  1.5  kw.  per  cubic  foot.  If,  however,  it  is  con- 
sidered that  part  of  the  charge  in  the  furnace  serves  as  a  heat-retain- 
ing wall,  and  the  calculation  is  limited  to  that  portion  of  the  charge 
which  is  converted  into  carborundum,  the  rate  of  heating  is  found  to 
be  3  kw.  per  cubic  foot. 

Collecting  the  results  obtained  above  for  the  power  required 
per  cubic  foot  of  electric  furnace,  the  following  general  figures 
may  be  given  for  moderate  or  large-sized  furnaces,  using  from  200  to 
1,000  h.p. :  Steel  melting  furnaces,  such  as  the  Heroult  and  Stassanb 
furnaces,  employ  5  to  8  kw.;  steel  melting  furnaces  such  as  the  Gin 
or  Kjellin  furnaces  employ  10  to  20  kw.;  ore  smelting  furnaces, 
such  as  those  of  Keller  and  Heroult,  employ  about  10  to  20  kw.  per 
cubic  foot  of  the  zone  of  fusion;  and  the  power  used  in  furnaces  of  the 
Acheson  type  varies  from  about  3  kw.  in  the  carborundum  furnace  to 
about  10  kw.  in  the  graphite  furnace.  Small-sized  furnaces  for  elec- 
tric smelting  may  employ  as  much  as  30  to  100  kw.  per  cubic  foot, 

1  F.  A.  J.  FitzGerald,  The  Ruthenburg  and  Acheson  Furnaces,  Electrochemical 
Industry,  vol.  iii,  p.  416. 

2  E.  G.  Acheson,  U.  S.  patent  711,031,  see  Electrochemical  Industry,  vol.  i, 
p.  130.     The  author  has  been  informed  by  the  Acheson  Graphite  Company  that 
these  dimensions  are  incorrect.     If  as  seems  reasonable  to  suppose  the  cross- 
section  for  a  75<D-kw  furnace  is  somewhat  larger  than  stated  above,  the  rate 
of  heating  would  be  proportionately  reduced,  and  would  agree  more  nearly  with 
the  other  figures  for  this  class  of  furnace. 

3  The  Carborundum  Furnace,  F.  A.  J.  FitzGerald,  Electrochemical  Industry, 
vol.  iv,  p.  53. 


132  THE  ELECTRIC  FURNACE 

and  Moissan  used  no  less  than  500    to  5,000    kw.    of    electrical 
power  per  cubic  foot  of  his  furnaces. 

VOLTAGE  REQUIRED  FOR  ELECTRIC  FURNACES 

Having  determined  how  many  watts  should  be  supplied  to  the 
furnace,  the  voltage  of  the  supply  must  next  be  considered.  The 
watts  supplied  are,  for  direct  current,  the  product  of  the  amperes 
and  the  volts,  while  for  alternating  current  they  are  somewhat  less; 
the  product  of  volts  and  amperes  being  multiplied  by  a  factor — • 
the  power- factor — which  varies  from  about  0.7  to  0.95  in  different 
forms  of  furnace,  in  order  to  obtain  the  watts.  The  heat  pro- 
duced depends  simply  upon  the  product  of  volts,  amperes  and 
power- factor,1  so  that  it  would  appear  possible  to  use  either  a 
high  or  low  voltage,  provided  the  watts  were  sufficient.  If  a 
moderate  current  at  a  high  voltage  could  be  employed,  it  would 
be  a  great  convenience,  but  this  is  usually  impracticable,  because 
it  is  not  generally  feasible  to  construct  a  furnace  having  a  suf- 
ficiently high  electrical  resistance. 

The  whole  problem  turns  upon  the  electrical  resistance  of  the 
resistor  R,  Fig.  25.  Suppose  that  a  furnace  needs  250  kw.  to  heat 
it,  then,  taking  direct  current  for  simplicity,  in  illustration,  if  the 
furnace  resistance  were  i  ohm,  a  500- volt  supply  would  drive  a 
current  of  500  amperes  through  the  furnace  and  would  develop  the 
necessary  250  kw.  If,  however,  the  furnace  had  a  resistance  of  only 
o.oi  ohm,  the  current,  in  amperes,  Vould  be  100  times  the  volts, 
and  5,000  amperes  at  50  volts  would  be  needed.  The  latter  case 
is  approximately  that  of  the  experimental  Heroult  furnace  used 
by  Dr,  Haanel,  and  shows  what  enormous  currents  will  have  to 
be  supplied  to  electric  smelting  furnaces,  if  constructed  on  any 
considerable  scale,  since  the  amperes  increase  with  the  size  of 
the  furnace  far  more  rapidly  than  the  volts.  The  use  of  such 
enormous  currents  is  inconvenient  and  increases  considerably  the 
cost  of  cables,  transformers,  electrodes  and  electrode  holders. 

Voltage  of  Arc -furnaces. — 'The  voltage  of  a  resistance  furnace 
is  nearly  proportional  to  the  current  flowing  through  it.  To  double 
the  current,  nearly  twice  the  voltage  would  be  needed;  but  in  an 
arc- furnace  (except  perhaps  in  the  Moissan  furnace,  which  is  so 
small  that  the  arc  fills  the  furnace)  the  voltage  does  not  increase 

1  Assuming  that  all  the  energy  is  converted  into  heat,  and  none  of  it  spent  in 
chemical  work,  such  as  electrolysis. 


THE  OPERATION  OF  ELECTRIC  FURNACES       133 

considerably  with  increase  of  current,  and  the  voltage  of  the  arc 
itself  is  often  less  as  the  current  increases.  This  sounds  impossible, 
but  it  is  a  well-established  fact,  and  points  to  the  instability  of  the  arc 
unless  a  steadying  resistor  is  placed  in  series  with  it.  In  a  large  fur- 
nace the  resistance  of  the  cables,  electrodes  and  transformer  or 
electric  generator  is  usually  sufficient  for  the  purpose,  but  the  writer 
has  frequently  extinguished  the  arc  in  an  experimental  furnace  by 
turning  on  too  much  current,  that  is  by  cutting  out  too  much  of 
the  regulating  rheostat,  and  so  applying  too  high  a  voltage  to  the 
arc.  The  resistance  of  an  arc  is  not  constant,  but  as  the  current 
increases  the  arc  becomes  larger  in  cross-section  and  its  resistance 
decreases  in  about  the  same  proportion,  or  even  faster  than  the  cur- 
rent increases;  the  voltage  in  consequence  remains  constant  or 
decreases. 

A  certain  minimum  voltage,  which  varies  from  about  25  to  35, 
is  needed  in  order  to  start  an  arc  at  all;  beyond  this  the  voltage 
increases  with  the  length  of  the  arc,  on  account  of  the  additional 
resistance  that  is  introduced  as  the  carbons  are  drawn  farther 
apart.  The  voltage  of  an  ordinary  lighting  arc  may  be  obtained 
by  the  formula: 


E  is  the  voltage,  /  is  the  length  of  the  arc  in  inches,  and  m  and  n 
are  constants,  which  for  good  pure  carbons  have  the  values  40.6 
and  40  respectively.1  The  constants  would  be  smaller  for  arcs 
between  cored  carbons,  for  arcs  enclosed  in  a  furnace,  so  that  the 
heat  of  the  arc  was  retained,  and  for  alternating-  current  arcs. 
The  following  figures  may  be  given  as  examples  of  direct-  current 
arcs  in  small  furnaces;  they  have  been  selected  from  the  work  of 
Henri  Moissan,  whose  furnace  is  shown  in  Fig.  6. 

TABLE  XIV—  VOLTAGE  OF  MOISSAN'S  ARC  FURNACE. 
Amperes  Volts  Amperes  Volts 

35-40  ..............   55  800  ..............    1  10 

ioo  .................   45  QOO  ..............     45 

250  .................    70,  75  1,000  ..............     50,  60,  70,  80,  no 

300  .................   60,  70,  85      1,200  ..............     70,  100,  no 

400  .................   80  2,000  ..............     60,  80,  100 

450  .................   60,  75  2,200  ..............     60,  70 

600  .................   60 

This  table  indicates  that  the  voltage  of  the  arc  is  not  determined 
by  the  amount  of  current  flowing  through  the  furnace,  but  depends 
1  Electric  Lighting,  by  F.  B.  Crocker,  vol.  ii,  p.  308. 


134  THE  ELECTRIC  FURNACE 

mainly  upon  the  length  of  arc  and  the  kind  of  vapor  present  in  the 
furnace.  The  length  of  arc  is  unfortunately  not  given,  but  prob- 
ably varied  from  about  half  an  inch  to  2  in.  or  3  in.;  aluminium 
vapor  is  mentioned  as  giving  a  long  arc  of  2  in.  to  2  1/2  in.  and  the 
2,000  and  2,200  ampere  arcs  at  60  volts  were  obtained  in  the  pres- 
ence of  iron  vapor.  The  actual  voltage  across  the  arc  will  be  some- 
what less  than  the  figures  given,  on  account  of  the  drop  of  volts 
along  the  electrodes  as  well  as  in  the  connections.  This  drop  is 
quite  considerable  in  the  case  of  heavy  currents,  and  would  vary 
from  about  5  to  20  or  even  30  volts  depending  on  the  current  and  the 
size  of  carbon  employed. 

Alternating  current  is  generally  used  in  arc  furnaces  intended 
for  industrial  use  and  the  Heroult  and  Stassano  steel  furnaces  may 
be  taken  as  examples. 

The  Stassano  furnace,  Fig.  109,  resembles  the  Moissan  furnace  in 
general  construction.  A  long  arc,  GH,  is  maintained  between  the 
ends  of  somewhat  slender  electrodes,  and  when  the  furnace  becomes 
thoroughly  hot,  the  arc  may  be  drawn  out  until  it  traverses  the  whole 
width  of  the  furnace.  In  one  furnace1  the  width  was  39  in.,  and 
an  alternating- current  arc  of  2,000  amperes  at  170  volts  was 
used.  It  will  be  seen  that  this  voltage  is  very  much  lower  than 
would  be  required  by  the -formula  given  above;  the  high  temperature 
of  the  furnace,  the  presence  of  metallic  and  other  vapors,  and  the 
use  of  alternating  instead  of  direct  current  all  contribute  to  this  effect. 

The  Heroult  furnace,  as  shown  in  Fig.  93,  resembles  a  Wellman 
tilting  open-hearth  furnace  from  which  the  gas  and  air  ports  have 
been  removed.  These  are  replaced  by  two  large  carbon  electrodes, 
C,  C,  which  enter  through  holes  in  the  roof.  The  furnace  is  basic 
lined,  but  it  would  be  possible  to  employ  an  acid  lining.  The  arc 
does  not  play  from  one  carbon  to  the  other,  as  in  the  Moissan  and 
Stassano  furnaces,  but  there  is  an  arc  between  each  electrode  and 
the  slag  and  metal  immediately  beneath  it.  In  this  way  the  heat 
of  the  arc  is  communicated  directly  to  the  metal,  and  as  two  arcs  are 
produced  in  series,  the  voltage  of  the  furnace  will  be  twice  as  great 
as  that  of  a  single  arc.  The  furnace  seen  by  the  Commission  at 
Kortfors  took  4,000  amperes  at  125  volts,  the  power  supplied  being 
about  450  kw.,2  while  the  smaller  furnace  at  La  Praz  took  nearly 
4,000  amperes  at  108  volts,  the  power  supplied  being  350  kw.3 

1  Electrochemical  Industry,  vol.  i  (1903),  p.  247. 

2  Dr.  Haanel,  European  Report,  1904,  p.  52. 

3  Dr.  Haanel,  European  Report,  1904,  p.  54. 


THE  OPERATION  OF  ELECTRIC  FURNACES        135 

The  voltage  of  each  arc  in  these  furnaces  will  be  about  45  or  55,  and 
the  arc  will  be  quite  short,  the  carbons  being  kept  just  clear  of  the 
slag. 

Voltage  of  Resistance  Furnaces.— Resistance  furnaces  have 
usually  a  lower  voltage  than  arc  furnaces  of  the  same  size.  The 
Heroult  ore  smelting  furnace,  Fig.  78,  is  of  the  resistance  type, 
as  no  arc  is  formed;  the  current  flowing  between  the  movable 
electrode  C,  and  the  carbon  lining  at  the  bottom  of  the  furnace, 
through  the  solid  and  liquid  materials  in  the  smelting  zone.  The 
electrical  resistance  of  these  materials  causes  the  energy  of  the 
current  to  be  converted  into  heat  and  largely  determines  the  volt- 
age of  the  furnace;  the  voltage  being  higher  for  a  given  current, 
if  the  contents  of  the  furnace  have  a  higher  electrical  resistance. 
In  the  experiments  with  this  furnace,  only  36  volts  were  required  to 
maintain  an  alternating  current  of  5,000  amperes.1 

The  Keller  ore  smelting  furnace,  Fig.  79,  is  equivalent  to  two 
Heroult  furnaces,  with  a  connecting  passage  between  the  cruci- 
bles of  the  two  furnaces.  This  passage  serves  as  a  reservoir  for 
the  molten  slag  and  iron,  and  also  serves  to  connect  electrically 
the  molten  metal  at  the  bottom  of  each  furnace:  an  alternative 
passage  for  the  current,  in  case  the  reservoir  were  emptied  at  any 
time,  is  provided  through  the  carbon  plugs  BE,  B1E1,  and  copper 
connector  EE1.  Electrically,  the  two  furnaces  are  arranged  in 
series,  the  current  being  supplied  through  the  two  movable  elec- 
trodes DA,  D1A1)  and  passing  in  series  through  the  two  smelting 
zones,  AB,  A1B1',  and  the  voltage  is  in  consequence  twice  as  great 
as  in  a  single  furnace  of  the  Heroult  type.  In  the  experiments 
made  by  Dr.  Haanel  at  Livet,2  the  double  shaft  furnace,  Nos.  n 
and  12,  took  a  current  of  11,000  amperes  at  59  volts,  and  the  double 
furnace,  Nos.  i  and  2,  took  7,250  amperes  at  55.3  volts. 

For  a  given  size  and  shape  of  furnace,  and  for  a  constant  distance 
between  the  electrodes  and  the  molten  iron  in  the  bottom  of  the 
furnace,  the  voltage  of  the  furnace  will  increase  with  the  current 
that  is  passed  through  it.  The  voltage  will  increase  less  rapidly 
than  the  current,  however,  because  at  the  higher  temperatures 
produced  by  the  increased  current,  the  electrical  resistance  of  the 
furnace  contents  will  be  less  than  it  was  with  the  smaller  current, 
and  so  the  ratio  of  voltage  to  current  will  be  reduced.  If  the  cross- 
section  of  the  furnace  were  increased,  so  that  the  current  density 

1  Dr.  Haanel,  Report  on  Experiments  at  Sault  Ste.  Marie,  1907,  p.  52. 

2  Dr.  Haanel,  European  Report,  1904,  p.  36. 


136  THE  ELECTRIC  FURNACE 

remained  constant,  i.e.,  the  number  of  amperes  for  each  square 
foot  of  cross-section  of  the  furnace  was  the  same  as  before,  the  volt- 
age would  remain  constant;  and  if  the  height  of  the  furnace  and  the 
distance  between  the  movable  electrode  and  the  bottom  of  the 
shaft  were  increased  proportionately,  the  voltage  would  increase  in 
the  same  ratio.  That  is  to  say,  in  furnaces  of  similar  shapes,  but 
of  different  dimensions  and  for  constant  current  densities,  the  volt- 
age will  be  proportional  to  the  linear  dimensions,  and  the  current 
will  be  proportional  to  the  square  of  these  dimensions.  It  follows 
from  this,  that  the  voltage  is  proportional  to  the  square  root  of  the 
current,  and  as  the  size  of  electric  furnaces  is  increased,  the  voltage 
necessary  to  operate  them  will  also  increase;  but  with  far  less 
rapidity  than  the  electrical  current  which  must  be  supplied.  In 
practice,  the  voltage  will  tend  to  increase  less  rapidly  than  the 
dimensions  of  the  furnace;  because  in  large  furnaces  the  same  cur- 
rent density  would  produce  a  rather  higher  temperature,  and  so 
would  make  the  charge  a  better  electrical  conductor,  or  smaller 
current  densities  could  be  employed  which  would  need  a  lower 
voltage. 

The  voltage  of  an  ore  smelting  furnace  of  the  Keller  or  Heroult 
type  depends  mostly  upon  the  height  to  which  the  electrode  is 
raised  from  the  bottom  of  the  furnace,  and  this  can  easily  be  changed 
during  the  smelting  operation,  thus  affording  a  convenient  means 
of  regulating  the  electric  current.  If  the  current  were  supplied  to 
such  a  furnace  at  an  absolutely  constant  voltage,  any  change  in 
the  resistance  of  the  furnace  would  lead  to  a  change  in  the  amount 
of  current,  the  voltage  remaining  constant;  and  in  running  a  fur- 
nace under  such  conditions,  the  electrode  would  be  lowered  to  in- 
crease the  current,  and  raised  to  decrease  it.  In  practice,  the  volt- 
age at  the  furnace  terminals  is  not  absolutely  constant,  but  de- 
creases with  an  increase  of  current,  on  account  of  the  resistance 
of  cables,  transformers,  etc.;  and,  in  consequence,  the  volts  and 
amperes  supplied  to  a  furnace  will  usually  vary  in  opposite  direc- 
tions. This  refers,  of  course,  to  changes  in  the  current  produced 
by  changes  in  the  furnace  itself;  external  changes  such  as  a  change 
in  the  speed  of  the  dynamo  supplying  the  current  would  reduce 
or  increase  both  the  volts  and  the  amperes.  The  drop  of  voltage 
that  accompanies  an  increase  of  current  is  not  objectionable  in 
electric  smelting,  and  it  serves  to  some  extent  as  an  automatic 
regulator  of  the  current. 


THE  OPERATION  OF  ELECTRIC  FURNACES        137 

CURRENT  DENSITY  IN  FURNACES 

After  considering  the  rate  of  heating  or  "power  density"  of  a 
furnace,  and  the  voltage  at  which  it  is  operated,  the  subject  of 
"current  density"  need  only  be  considered  briefly.  The  current 
density  in  a  furnace  will  usually  vary  considerably  from  point  to 
point,  except  where  the  current  flows  through  a  resistor  of  regular 
dimensions.  In  some  smelting  furnaces  the  current  may  be  dis- 
tributed so  evenly  through  the  contents  of  the  furnace,  that  we  can 
take  the  current  density  as  the  number  of  amperes  divided  by  the 
internal  cross-section  of  the  furnace  measured  at  right  angles  to 
the  direction  of  the  current.  In  the  Heroult  ore  smelting  furnace, 
Fig.  78,  the  cross-section  at  the  level  D  is  5.4  sq.  ft.,  and  the  current 
5,000  amperes,  giving  an  average  current  density  of  930  amperes 
per  square  foot,  or  nearly  6  1/2  amperes  per  square  inch.  The  Keller 
ore  smelting  furnace  at  Livet  had  a  current  density  of  10  or  12 
amperes  per  square  inch  of  cross-section.  In  the  Acheson  carbor- 
undum furnace  the  current  density,  according  to  published  accounts, 
varies  from  10  to  30  amperes  per  square  inch  of  the  core,  and  in 
the  Kjellin  induction  furnace,  there  may  be  as  much  as  600  amperes 
per  square  inch  of  cross-section  of  the  molten  metal. 

"Pinch  Effect." — The  current  density  in  molten  conductors  is 
limited  by  a  curious  phenomenon,  which  was  first  investigated  by 
Dr.  Hering.1  In  operating  furnaces  of  the  Kjellin  or  the  Gin  type, 
if  the  current  density  is  increased  beyond  a  certain  amount  the 
surface  of  the  metal  becomes  depressed  at  some  point,  and  the  de- 
pression may  increase  until  it  divides  the  conductor,  and  thus 
interrupts  the  current.  It  is  well  known  that  parallel  conductors 
attract  each  other  when  electric  currents  flow  through  each  in  the 
same  direction;  and  the  same  force  urges  together  the  particles  of 
a  molten  conductor  carrying  a  current.  If  this  conductor  is  cylin- 
drical, all  the  particles  of  which  it  is  composed  tend  to  move  toward 
the  axis,  but  as  the  resulting  pressure  at  the  axis  is  equal  at  every 
point  along  the  length  of  the  cylinder,  no  apparent  effect  is  produced. 
If  however  the  conductor  is  initially  somewhat  smaller  in  cross- 
section  at  one  point  than  elsewhere,  the  current  density  here  will 
be  greater,  and  the  radial  forces  will  therefore  be  greater,  producing 

1  Carl  Hering,  "A  Practical  Limitation  of  Resistance  Furnaces;  the  'Pinch' 
Phenomenon."  Trans.  Am.  Electrochem.  Soc.,  xi,  1907,  p.  329.  "The  Working 
Limit  in  Electrical  Furnaces  Due  to  the  'Pinch*  Phenomenon."  Trans.  Am. 
Electrochem.  Soc.,  xv,  1909,  p.  255. 


138  THE  ELECTRIC  FURNACE 

a  greater  pressure  at  the  axis  and  therefore  causing  the  conducting 
liquid  to  move  along  the  axis  away  from  the  point  of  small  cross- 
section.  This  causes  a  pinching  of  the  conductor  at  this  point. 

The  pressure  produced  at  the  axis  of  a  cylindrical  liquid  con- 
ductor, by  the  electro-magnetic  attraction  of  the  particles  com- 
posing it,  has  been  calculated  by  Dr.  Northrup  and  is  given  by  the 
formula:1 


44,479>iooS 

in  which  P  is  the  pressure  at  the  axis  in  pounds  per  square  inch, 
C  is  the  total  current  (either  direct  or  alternating)  in  amperes,  and 
S  is  the  cross-section  in  square  inches. 

In  a  conductor  of  varying  cross-section,  the  value  of  P  varies 
from  point  to  point  along  the  axis,  and  this  causes  an  axial  flow  of 
the  liquid  from  points  of  small  to  points  of  large  cross-section;  the 
liquid  so  displaced  returning  along  the  surface  of  the  conductor. 
This  circulation,  which  has  been  termed  the  "jet  effect,"  is  an 
essential  feature  of  the  Hering  furnace,  Fig.  21. 

If  the  molten  conductor  lies  in  an  open  channel,  as  in  the  Kjellin 
furnace,  the  attraction  between  the  conducting  particles,  tends  to 
depress  the  surface  of  the  metal  at  any  point  of  restricted  cross- 
section.  This  action,  if  not  too  strong,  may  be  balanced  by  the 
hydraulic  pressure  tending  to  keep  the  liquid  level;  but  if  the 
depression  amounts  to  a  certain  proportion  of  the  original  depth  of 
the  liquid,  the  equilibrium  between  these  forces  becomes  unstable, 
as  at  this  point  any  further  decrease  in  cross-section  increases  the 
pinching  force  (by  raising  the*  current  density)  more  rapidly  than  it 
increases  the  restoring  force  due  to  difference  of  level,  and  the  con- 
ductor suddenly  breaks  in  two. 

Dr.  Hering  finds  that  for  a  conductor  of  rectangular  cross-section 
this  critical  condition  should  be  reached  when  the  depth  of  the  liquid 
has  been  reduced  to  half  its  original  amount.  Taking  a  conductor 
having  originally  a  depth  equal  to  twice  its  width,  so  that  with  the 
critical  current  the  pinched  portion  will  be  square  in  section,  Dr. 
Hering  offers  the  formula: 

C£>  =  8< 

1  In  the  original  formula,  as  modified  by  Dr.  Hering,  Trans.  Am.  Electrochem. 
Soc,  xv,  1909,  p.  257.  S  stands  for  square  centimeters  and  P  for  pounds  per 
square  centimeter.  The  formula  is  unaffected  by  this  change  from  square 
centimeters  to  square  inches. 


THE  OPERATION  OF  ELECTRIC  FURNACES        139 

in  which  CD  is  the  critical  current  density  in  amperes  per  square 
inch  of  the  original  section,  G  is  the  specific  gravity  of  the  molten 
metal,  and  H  is  the  depth  of  the  metal  in  inches. 

In  the  case  of  an  induction  furnace  having  a  ring  of  molten  iron 
(S.G.  =  6.88)  10  in.  X  5  in.  in  cross-section,  the  critical  current  density, 
above  which  pinching  would  take  place,  would  be  742  amperes  per 
square  inch  of  the  original  section,  or  a  total  current  of  37,100 
amperes. 

REGULATION  OF  ELECTRIC  FURNACES 

Under  this  general  heading  may  be  considered  the  regulation 
of  the  electrical  power  supplied  to  a  furnace,  the  regulation  of  the 
electrodes,  and  the  supply  and  removal  of  the  material  to  be  treated. 

The  regulation  of  the  power  may  be  effected  by  means  of  the 
electrodes  themselves  as  in  the  Heroult  and  other  steel  furnaces, 
the  electrodes  being  adjusted  so  that  the  current  through  each  is 
kept  constant,  or  that  a  constant  voltage  is  maintained  between 
each  electrode  and  the  contents  of  the  furnace.  This  is  usually 
effected  by  electric  motors  which  raise  or  lower  the  movable  elec- 
trodes. The  motors  are  started,  stopped,  and  reversed,  by  instru- 
ments operated  by  the  voltage  of  the  furnace,  in  such  a  manner  as 
to  keep  this  constant.  In  the  Keller  furnace,  Fig.  79,  and  the  Her- 
oult steel  furnace,  Fig.  93,  there  are  two  movable  electrodes;  each  of 
these  being  independently  regulated  so  as  to  keep  a  constant  voltage 
between  itself  and  the  molten  metal  in  the  furnace.  The  auto- 
matic regulating  apparatus  for  the  Heroult  furnace  is  described  in 
Dr.  Haanel's  report.1 

The  change  in  electrical  resistance  due  to  a  change  in  the  height 
at  which  the  electrode  is  kept  in  a  smelting  furnace,  affords  a  means 
of  adapting  the  furnace  to  a  variety  of  voltages.  Electrically,  it 
is  advantageous  to  operate  the  furnace  at  as  high  a  voltage  as  it  will 
take,  and  it  is,  therefore,  important  to  ascertain  how  high  the  elec- 
trode can  be  raised  without  causing  trouble  in  the  furnace.  The 
exact  height  that  is  most  desirable  will  depend  upon  a  number 
of  factors,  such  as  the  shape  of  furnace,  size  of  electrode,  nature  of 
the  charge,  and  amount  of  current;  but  the  distance  between  the 
electrode  and  the  molten  slag  in  a  shaft  smelting  furnace  should 
probably  be  less  than  the  width  of  the  crucible  of  the  furnace. 

In  large  electrical  smelting  furnaces,  such  as  the  iron  furnaces  at 
Domnarfvet  and  Trollhattan,  see  Figs.  90  and  92,  the  regulation 

1  Dr.  Haanel,  European  Report,  1904,  p.  6. 


140  THE  ELECTRIC  FURNACE 

is  not  effected  by  means  of  the  electrodes,  which  are  only  moved 
at  long  intervals  as  they  wear  away,  but  by  a  regulating  device  which 
controls  the  voltage  of  the  electrical  supply,  see  Fig.  49.  This 
device  raises  or  lowers  the  voltage,  as  the  resistance  of  the  furnace 
changes,  so  as  to  maintain  a  constant  supply  of  power. 

In  other  types  of  resistance  smelting  furnaces,  the  current  passes 
through  the  molten  slag  and  metal,  instead  of  through  the  melting 
ore,  the  current  entering  by  means  of  two  or  more  carbon  electrodes 
which  dip  into  the  fused  slag,  as  in  the  Harmet  furnace,  Fig.  80;  by 
electrodes  of  fused  metal  lying  beneath  the  slag,  as  in  the  Laval 
furnace,  Fig.  20;  or  by  induction,  without  the  use  of  electrodes,  as 
in  the  Snyder  furnace,  Fig.  131.  In  such  furnaces,  the  slag  becomes 
heated  above  its  melting  temperature,  by  the  passage  of  the  current, 
and  melts  or  dissolves  the  ore  which  rests  upon  it.  The  voltage 
depends  upon  the  shape  and  size  of  the  furnace,  but  on  account  of 
the  low  specific  resistance  of  molten  slags  it  will  usually  be  lower 
than  in  furnaces  in  which  the  current  passes  through  the  melting 
ore,  as  well  as  through  the  fused  slag.  The  molten  metal  accumulat- 
ing in  the  bottom  of  the  furnace  will  also  tend  to  lower  the  voltage, 
by  carrying,  on  account  of  its  greater  conductivity,  a  large  part  of  the 
current.  It  is  not  practicable  to  regulate  such  furnaces  by  moving 
the  electrodes,  and  the  regulation  must  be  effected  by  adjusting  the 
voltage  of  the  supply  as  explained  above. 

In  the  Kjellin  and  Gin  furnaces,  the  electrical  resistance  of 
the  steel  itself  is  relied  upon  for  converting  the  energy  of  the  current 
into  heat.  The  specific  resistance,  or  resistivity,  of  steel,  even  when 
molten,  is  so  small  that  the  metal  must  be  contained  in  a  trough  or 
canal  of  considerable  length  and  moderate  cross-section,  in  order 
to  have  any  appreciable  electrical  resistance;  and  even  then,  the 
voltage  is  very  small,  and  enormous  currents  must  be  supplied,  in 
order  to  heat  the  furnace.  In  the  Kjellin  furnace,  already  referred 
to,  a  current  of  30,000  amperes  is  supposed  to  circulate  around  the 
ring  of  molten  steel;  the  force  required  to  drive  such  a  current 
being  only  7  volts.  In  the  Gin  furnace,  the  voltage  is  also  very 
small;  15  volts  maintaining  currents  that  range  from  10,000  to 
100,000  amperes.  Furnaces  of  such  low  resistance  are  very  unsatis- 
factory electrically;  but  the  absence  of  carbon  electrodes,  and  the 
production  of  the  heat  directly  in  the  molten  steel,  render  them  very 
suitable  for  steel-making. 

In  the  Acheson  furnaces,  the  resistor  consists  of  a  special  core  of 
carbon,  surrounded  by  the  charge,  or  the  charge  itself  is  the  resistor. 


THE  OPERATION  OF  ELECTRIC  FURNACES        141 

In  either  case,  the  resistor  remains  solid  during  the  operation,  and 
cannot  be  lengthened  or  shortened  in  order  to  regulate  the  current. 
Moreover,  as  the  furnace  is  intermittent  in  action,  the  temperature 
of  the  resistor  is  not  constant,  as  in  a  smelting  furnace,  but  rises 
continuously  during  the  run.  This  rise  of  temperature  reduces 
very  considerably  the  resistance  of  the  furnace,  and  hence,  the 
relation  between  the  volts  and  the  amperes.  For  example,  if  the 
furnace  had  a  core  of  coke,  the  resistance  would  fall  to  about  one- 
half  its  original  value  when  the  furnace  became  thoroughly  hot, 
and  if  the  heat  were  sufficient  to  graphitize  the  coke,  the  resistance 
would  fall  still  further,  the  resistance  of  the  heated  graphite  being 
only  about  one-sixth  of  that  of  the  cold  coke  from  which  it  was 
originally  produced.  Such  a  furnace  would  be  very  difficult  to 
operate  with  a  constant  voltage  supply;  because  if  it  were  propor- 
tioned so  as  to  draw  a  suitable  current  when  heated,  the  current 
that  would  flow  through  the  cold  furnace  would  be  so  small  (only 
one-sixth  of  the  final  current),  that  the  furnace  would  heat  up  very 
slowly,  and  the  consumption  of  power  would  change  very  consider- 
ably during  the  run.  The  price  paid  for  electrical  energy  is  usually 
based  upon  the  maximum  rate  at  which  it  will  be  used,  and  a 
furnace  which  only  used  15  to  25  per  cent,  of  its  maximum  power 
for  a  large  proportion  of  the  run,  would  be  very  inefficient  financially. 
It  is  necessary,  therefore,  to  change  the  voltage  of  the  supply  during 
the  run,  and  for  this  purpose  a  special  induction  regulator  has  been 
devised,  which  will  change  the  voltage  from  about  200  volts  at  the 
beginning  of  the  run  to  80  volts  at  the  end  of  the  run,  maintaining 
the  same  power  (about  1,000  h.p.)  all  the  time.1  More  recently 
the  voltage  regulator,  shown  in  Fig.  49,  has  been  used  for  operating 
the  carborundum  furnaces  at  Niagara  Falls. 

It  will  be  noticed  that  the  change  in  the  voltage  is  less  than 
the  change  in  the  resistance  of  the  furnace.  This  follows  directly 
from  the  relationship  between  volts,  amperes,  and  watts,  because 
(omitting  any  consideration  of  inductance),  the  voltage  must  vary, 
for  constant  power,  as  the  square  root  of  the  resistance  of  the 
furnace.  Thus,  if  P  is  the  power  in  watts,  E  the  voltage,  /  the 
current* in  amperes,  and  R  the  resistance  in  ohms: 

E  E2 

P  =  EI,  and  7=  ^>  therefore  P  =  ^ 

or,  for  constant  power,  E  must  vary  as  the  square  root  of  R. 

1 F.  A.  J.  FitzGerald,  Miscellaneous  Accessories  of  Resistance  Furnaces, 
Electrochem.  Industry,  vol.  in,  p.  n. 


142  THE  ELECTRIC  FURNACE 

• 

The  means  employed  for  the  regulation  of  electrodes  depends 
mainly  upon  whether  this  regulation  serves  to  control  the  supply  of 
power  or  whether  it  is  merely  needed  to  advance  the  electrodes  as 
they  wear  away.  In  the  former  case  automatic  regulation  is  usually 
employed,  as  in  the  Heroult  steel  furnace  and  many  carbide  furnaces, 
while  in  the  latter  case  the  regulation  is  usually  by  hand;  some 
simple  mechanism  being  used  to  give  the  necessary  power,  as  in  the 
furnaces  at  Domnarfvet  and  Trollhattan.  Some  exceptions  to  this 
general  rule  are  the  Stassano,  Keller,  and  Girod  steel  furnaces,  in 
which  the  regulation  of  the  electrodes  is  by  hand,  although  this 
regulation  serves  also  to  control  the  current. 

The  regulation  of  electric  furnaces  by  the  supply  and  removal 
of  the  material  that  is  being  treated,  which  may  thus  be  given  a 
shorter  or  longer  time  in  the  furnace,  varies  too  much  in  individual 
cases  to  be  considered  generally.  In  intermittent  furnaces  of  the 
Acheson  type,  the  charge  is  placed  in  the  furnace,  which  is  then 
heated,  allowed  to  cool,  and  finally  discharged.  In  such  cases  the 
length  of  the  operation  will  affect  the  character  of  the  product  and 
this  will,  in  general,  be  determined  by  experience,  although  in  some 
cases  an  observation  of  the  furnace  temperature  may  assist  in 
determining  when  the  operation  is  complete.  In  other  furnaces, 
such  as  the  Willson  carbide  furnace,  the  charge  is  introduced 
continually  by  a  mechanical  device,  until  the  furnace  is  full;  when  it 
is  allowed  to  cool  before  being  discharged.  It  is  generally  desirable 
in  electric  furnaces  to  have  the  charging  and  discharging  as  nearly 
continuous  as  practicable.  A  continuous  movement  of  the  charge 
through  the  furnace  takes  place  in  nitric-acid  furnaces,  and  in 
certain  other  furnaces  in  which  the  material  to  be  heated  is  a  rod 
or  strip  which  is  moved  continuously  through  the  furnace.  In 
this  case  the  speed  at  which  the  material  is  moved  will  determine 
the  time  during  which  it  is  subjected  to  the  heat  of  the  furnace. 
In  smelting  furnaces  a  charge  may  be  added  in  weighed  amounts 
at  short  intervals,  or  may  be  fed  continuously  by  charging  machin- 
ery, while  the  molten  product,  usually  metal  and  slag,  is  tapped  out 
at  intervals.  In  such  furnaces  the  charging  and  tapping  must  be 
carried  out  in  accordance  with  the  rate  of  smelting  of  the  furnace, 
and  these  have  no  regulating  effect  on  the  furnace  operation. 

MEASUREMENT  OF  FURNACE  TEMPERATURES 

In  many  furnace  operations  it  is  very  important  to  be  able  to 
measure  the  temperature  attained,  and  pyrometry,  or  the  measure- 


THE  OPERATION  OF  ELECTRIC  FURNACES        143 

ment  of  high  temperatures,  has  developed  rapidly  during  recent 
years. 

Although  many  electrical  furnaces  operate  at  so  high  a  tempera- 
ture that  no  pyrometer  can  be  placed  in  them,  a  number  of  furnaces 
are  now  used  for  smelting  purposes  at  comparatively  low  tempera- 
tures, below,  for  example,  1,500°  C,  or  1,600°  C, 
and  for  such  furnaces  several  forms  of  pyrometer  are  suitable.1 
A  few  of  the  more  important  may  be  briefly  described. 

Electrical  Resistance  Pyrometer. — The  Callendar  resistance 
pyrometer2  contains  a  coil  of  platinum  wire  carefully  insulated  and 
protected  from  the  furnace  gases,  and  so  arranged  that  its  electrical 
resistance  can  be  accurately  measured.  The  resistivity  of  pure 
metals  increases  very  regularly  with  the  temperature,  and  accurate 


FIG.  58. — Electrical  resistance  pyrometer. 

temperature  measurements  can  be  made  in  this  manner  up  to 
1,000°  C,  or  1,100°  C.  This  pyrometer,  as  shown  in  Fig.  58,  con- 
sists of  a  coil  of  fine  platinum  wire,  C,  wound  on  a  framework  of 
mica,  M,  and  having  leads  of  stouter  platinum  wire,  L.  The  cool 
ends  of  these  leads  are  attached  to  binding  posts,  P,  from  which 
copper  wires  lead  to  instruments  for  measuring  the  electrical  resist- 
ance of  the  coil  of  platinum  wire.  The  resistance  measured  includes 
that  of  the  platinum  leads,  which  will  be  partly  in  the  furnace  and 
partly  outside.  In  order  to  avoid  the  inaccuracy  resulting  from  this, 
duplicate  leads,  R,  are  provided,  which  are  short-circuited  at  the  hot 

1  High-temperature  Measurements,"  by  le  Chatelier  and  Boudouard,  trans, 
by  C.  K.  Burgess. 

Pyrometers  suitable  for  Metallurgical  Work.  Journ.  Iron  and  Steel  Inst.  i, 
1904. 

Methods  of  Pyrometry,  C.  L.  Waidner,  Proc.  Eng.  Soc.  of  Western  Pa.,  1904, 
p.  98. 

Seger  Cones,  Hofman  and  Demond.  Trans.  Amer.  Inst.  of  Mining  Engineers, 
vol.  xxiv,  p.  42,  and  xxix,  p.  682. 

2  Technical  Thermometry,  a  pamphlet  issued  by  the  Cambridge  Scientific 
Instrument  Co.,  1906. 

Callendar,  Phil.  Mag.,  vol.  xlvii,  1899,  pp.  191  and  519. 

Chappuis  and  Marker,  Phil.  Trans.  Roy.  Soc.  A.,  vol.  cxciv,  1900,  pp.  37-134. 


144  THE  ELECTRIC  FURNACE 

end,  and  the  measuring  instruments  are  arranged  to  show  the  differ- 
ence of  resistance  between  the  coil  of  platinum  wire  with  its  leads, 
L,  and  the  duplicate  leads,  R.  This  difference  represents  accurately 
the  resistance  of  the  platinum  coil  alone,  and  so  indicates  the  tem- 
perature of  the  furnace.  Mica  is  used  to  support  the  platinum 
wire  coil  and  leads,  as  it  remains  an  insulator  at  high  temperatures. 
The  whole  is  enclosed,  for  protection,  in  a  tube,  T,  of  porcelain  or 
fused  quartz.  * 

Thermo-electric  Pyrometer.1 — This  consists  of  two  wires  of  dif- 
ferent metals.  These  are  fused  or  twisted  together  at  one  end,  which 
is  placed  in  the  furnace,  while  the  other  ends  of  the  wires  are  connected 
to  a  galvanometer  or  instrument  for  measuring  a  very  small  electric 
current.  When  the  junction  of  the  wires  is  heated,  a  small  electric 
current  is  generated  and  in  this  way  the  temperature  of  the  furnace 
can  be  measured.  The  indications  of  this  instrument  are  somewhat 
less  accurate  than  those  of  the  platinum  resistance  pyrometer,  but 


FIG.  59. — Thermo-electric  pyrometer. 

the  thermo-couple  can  be  used  up  to  1,700°  C.,  and  in  many  ways  is 
more  convenient  than  the  resistance  pyrometer.  For  very  high 
temperatures  the  wires  are  composed  of  platinum  and  an  alloy  of 
platinum  with  rhodium  or  iridium,  but  for  lower  temperatures 
cheaper  metals  can  be  used. 

The  thermo-electric  pyrometer  shown  in  Fig.  59  consists  of  a  wire 
of  platinum,  P,  and  a  wire  of  platinum  alloyed  with  10  per  cent,  of 
rhodium  or  iridium,  R.  These  wires  are  insulated  from  each  other, 
and  protected  from  the  contents  of  the  furnace  by  tubes  of  fused 
quartz,  T,  and  their  ends  are  twisted  and  fused  together  at  H.  The 
other  end  of  each  wire  is  soldered  at  C  to  a  copper  wire  leading  to  a 
galvanometer,  G.  The  junction  H  is  placed  in  the  furnace  whose 
temperature  is  to  be  measured,  and  the  junctions  C  are  kept  at  a 
constant  temperature  by  a  stream  of  cold  water.  The  deflection  of 

1  Barus.  Bull,  U.  S.  Geol.  Survey,  No.  54,  Washington,  1889. 
Roberts- Austen,  Trans.  Amer.  Inst.  of  Mining  Engineers,  1893. 
Stansfield,  Phil.  Mag.,  xlvi,  1898,  p.  59. 


THE  OPERATION  OF  ELECTRIC  FURNACES        145 

the  galvanometer  depends  on  the  difference  of  temperature  between 
H  and  C.  The  pyrometer  is  calibrated  empirically  by  noting  the 
deflection  produced  at  certain  known  temperatures,  such  as  the 
melting-points  of  pure  metals.  The  galvanometer  must  have  a  fairly 
high  resistance,  so  as  to  render  unimportant  any  small  changes  in 
the  resistances  of  the  wires  P  and  R  and  other  parts  of  the  system. 
The  action  of  the  pyrometer  depends  on  the  change  in  thermo- 
electric force  between  the  metals  P  and  R,  due  to  change  of  tempera- 
ture; and  for  accurate  work  the  galvanometer  would  be  replaced  by  a 
potentiometer,  or  other  instrument  better  adapted  to  the  measure- 
ment of  small  electromotive  forces. 

This  pyrometer  is  limited  by  the  melting  of  the  wire  which  takes 
place  at  about.  1,7 50°  C.  For  still  higher  temperatures  the  author 
has  devised  a  thermo-electric  pyrometer  depending  on  the  electro- 
motive force  between  amorphous  carbon  and  graphite.  The  difficul- 
ties of  construction  in  these  materials  are  considerable  and  they  have 
been  overcome,  as  shown  in  Fig.  60,  by  the  use  of  a  graphite  tube 


FIG.  60. — Graphite-carbon  pyrometer. 

closed  at  one  end  (obtained  by  boring  a  graphite  electrode)  and 
placing  within  this  tube  a  rod  of  amorphous  carbon,  which  makes 
contact  only  at  the  closed  end  of  the  tube.  A  water-cooled  jacket 
is  attached  to  the  open  end  of  the  graphite  tube  and  within  this 
contact  is  made  to  the  carbon  rod  by  means  of  an  insulated  terminal. 
This  pyrometer  can  be  used  up  to  temperatures  at  which  carbon 
is  converted  to  graphite,  that  is  to  say  nearly  up  to  the  temperature 
of  the  electric  arc.  It  is  especially  suitable  for  use  in  furnaces  that 
contain  carbon  and  are  free  from  oxidizing  gases.  In  the  presence 
of  air  it  would  be  necessary  to  protect  the  graphite  tube  with  an  outer 
tube  of  some  refractory  material,  such  as  carborundum  or  alundum, 
and  this  would,  of  course,  limit  the  range  of  the  pyrometer.  In  using 
this  pyrometer  it  is  essential  that  the  cooler  end,  at  which  it  connects 
to  the  copper  wires  leading  to  the  measuring  instrument,  shall  be 
kept  very  carefully  at  a  constant  temperature,  as  changes  in  this 
temperature  affect  the  indication  far  more  than  equal  changes  in  the 
10 


146  THE  ELECTRIC  FURNACE 

furnace  temperature.  The  carbon  rod  is  supported  only  at  its  cool 
end  and  at  its  point  of  contact  with  the  graphite  tube;  thus  avoiding 
the  use  of  any  insulating  substance  in  that  part  of  the  pyrometer 
wnich  is  exposed  to  high  temperature.  This  is  essential  as  all  sub- 
stances become  conductors,  even  if  they  do  not  fuse,  at  very  high 
temperatures. 

The  author  has  not  had  the  opportunity  of  making  an  extended 
study  of  this  pyrometer,  but  the  work  already  done  indicates  that  it 
should  have  a  considerable  field  of  usefulness.  Difficulty  is  met  in 
obtaining  samples  of  graphite  and  amorphous  carbon  having 
constant  thermo-electric  properties  and  it  must  be  remembered  that 
at  electric-furnace  temperatures,  there  will  be  danger  of  the  gradual 
transformation  of  the  carbon  into  graphite,  and  this  .will  necessitate 
frequent  recalibration  of  the  pyrometer. 

Pyrometers  of  the  thermo-electric  type  can  be  made  to  record 
their  indications  by  means  of  a  moving  photographic  plate  on  which 
a  ray  of  light  falls  from  the  mirror  of  the  indicating  galvanometer. 
There  are  also  a  number  of  commercial  forms  of  recording  pyrometers 
which  produce  an  ink  record  of  the  temperature. 

Optical  or  Radiation  Pyrometers. — These  depend  on  the  measure- 
ment of  the  amount  or  the  color  of  the  light  emitted  by  a  heated 
substance,  or  of  the  amount  of  heat  which  is  radiated.  They  can 
be  used  to  measure  the  temperature  of  the  hottest  furnaces,  as  no 
part  of  the  instrument  need  be  inserted  in  the  furnace.  These  pyro- 
meters can  be  sighted  on  the  heated  contents  of  a  furnace,  through 
an  opening  in  the  wall,  or  on  the  closed  inner  end  of  a  tube  entering 
the  furnace. 

Optical  pyrometers1  are  of  several  types.  One  form  depends  on 
the  measurement  of  the  color  of  the  light  emitted  by  the  furnace, 
depending  on  the  fact  that  the  emitted  light  changes  its  color  with 
the  temperature. 

The  Mesure  and  Nouel  pyrometer  is  of  this  kind  having  a  pair  of 
Nicol  prisms  and  a  quartz  plate  so  arranged  that  a  different  reading 
is  obtained  for  different  colored  light. 

A  number  of  pyrometers  measure  the  brightness  of  the  light, 
preferably  the  brightness  of  some  particular  colored  light  emitted 
by  the  furnace,  red  light,  for  example. 

Of  these  may  be  mentioned  the  Cornu  -  le  Chatelier  pyrometer 

Optical  Pyrometry,  by  C.  W.  Waidner  and  G.  K.  Burgess,  Bull.  No.  2, 
Bureau  of  Standards,  Washington,  1905. 

Radiation  Pyrometry,  G.  A.  Shook,  Met.  and  Chem.  Eng.,  x,  1912,  p.  238. 


THE  OPERATION  OF  ELECTRIC  FURNACES        147 


which  compares  the  brightness  of  the  furnace  with  that  of  a  standard 
oil  lamp,  varying  the  amount  of  light  received  from  the  furnace  by 
means  of  an  adjustable  diaphragm  until  equality  is  obtained. 

A  recent  modification  of  this  is  the  Shore  pyrometer. 

The  Morse  pyrometer  compares  directly  the  brightness  of  the 
furnace  with  that  of  a  carbon  filament  heated  by  an  electric  current. 
The  amount  of  current  required  to  produce  equality  of  brightness 
is  noted  and  this  affords  a  measure  of  the  temperature. 

The  Wanner  pyrometer  compares  the  brightness  of  the  furnace 
with  that  of  a  standard  electric  lamp;  the  comparison  between  this 
and  the  furnace  being  effected  by  an  elaborate  optical  system. 

Another  type  of  pyrometer  measures  the  total  intensity  of  the 
heat-radiation  from  the  furnace;  the  heat  being  allowed  to  fall  on 
a  minute  thermo-couple,  in  the  pyrometer. 

The  best  known  of  these  pyrometers  is  the  Fery  pyrometer. 
This  instrument  is  a  telescope,  see  Fig.  61,  having  an  objective  lens 


FIG.  61. — Fery  radiation  pyrometer. 

A,  preferably  made  of  fluorite;  or  a  mirror,  m.  The  radiant  heat 
is  concentrated  by  the  lens  or  mirror  on  the  junction  of  a  minute 
thermo-couple,  which  is  connected  to  insulated  binding  screws,  and 
from  these  to  a  galvanometer  or  millivoltrneter.  The  "  cold-junc- 
tion" of  the  thermo-couple  is  protected  from  the  radiation  by  screens, 
and  the  difference  of  temperature  between  the  hot  and  cold  junc- 
tions, and  hence  the  reading  on  the  millivoltrneter,  will  depend  on 
the  amount  of  radiant  heat  falling  on  the  hot-junction,  and  there- 
fore on  the  temperature  of  the  furnace.  The  thermo-couple  is  very 
small,  so  that  there  is  scarcely  any  lag  in  its  indications.  The  lens 
or  mirror  can  be  moved  so  as  to  form  the  image  of  the  furnace  on 
the  thermo-couple  with  the  aid  of  the  eye-piece,  E.  The  change 
in  focus  of  the  lens  or  mirror,  depending  on  the  distance  between 
the  instrument  and  the  furnace,  would  introduce  an  error  in  the 
reading,  but  this  error  is  avoided  by  the  diaphragm  F  in  the  first 
figure  and  a  corresponding  diaphragm  in  the  second  figure. 


148 


THE  ELECTRIC  FURNACE 


Simpler  forms  of  this  pyrometer  are  the  Thwing  and  the 
Foster's  fixed  focus  pyrometer. 

The  gases  and  vapors  present  in  electric  furnaces  introduce  ser- 
ious difficulties  into  the  use  of  optical  and  radiation  pyrometers. 
It  is  often  impossible  to  keep  an  opening  into  the  furnace  for  the 
observation  of  its  temperature,  and  a  closed  tube  is  introduced  as 
shown  in  Fig.  62;  the  pyrometer  being  sighted  on  the  inner  closed 
end,  marked  black  in  the  figure.  Even  with  this  precaution  the 
tube  becomes  obscured  by  vapors,  and  Gillett,1  employed  an  inner 
tube,  open  at  both  ends,  as  shown  in  the  figure,  and  withdrew  the 
gases  from  the  space  between  the  tubes,  thus  leaving  the  inner  tube 
clear  for  observation.  The  tubes  can  be  made  of  carbon,  carborun- 


FIG.  62. — Gillett  tube  in  carborundum  furnace. 

dum,  or  other  material  according  to  the  nature  of  the  furnace.  In 
Fig.  62  the  tube  is  used  in  measuring  the  temperature  of  the  core  of 
a  carborundum  furnace. 

Other  methods  of  measuring  electric  furnace  temperatures  con- 
sist in  placing  in  the  furnace  a  piece  of  carbon  or  of  some  other  re- 
fractory material  and  noting  at  the  end  of  the  operation  what 
change  has  taken  place  in  the  material  used. 2  In  the  case  of  carbon 
the  specific  gravity  increases  with  the  temperature  to  which  it  has 
been  exposed,  but  the  change  depends  upon  the  time  during  which 
it  has  been  heated  as  well  as  upon  the  highest  temperature  attained, 
and  the  indications  of  such  a  test  are  difficult  to  convert  into  degrees 
of  temperature.  In  some  cases  the  temperature  of  an  electric  fur- 

1L.  E.  Saunders,  "Temperature  Measurements  on  the  Silicon  Carbide  Fur- 
nace," Am.  Electrochem.  Soc.,  xxi,  1912,  p.  425. 

2  F.  A.  J.  FitzGerald,  Trans.  Amer.  Electrochem.  Soc.,  vol.  vi,  p.  31. 


THE  OPERATION  OF  ELECTRIC  FURNACES        149 


nace  can  be  determined  from  the  amount  of  electrical  energy  sup- 
plied to  it.  Thus  Mr.  W.  C.  Arsem,  in  working  with  a  small  vacuum 
electric  furnace,  observed  how  much  power  was  needed  to  maintain 
the  furnace  at  three  lower  temperatures,  which  could  be  measured, 
and  then  deduced  by  means  of  a  curve  the  temperature  that  should 
be  produced  by  any  other  amount  of  electric  power.1 

This  section  may  be  concluded  by  a  table  of  temperatures  pub- 
lished recently  by  G.  K.  Burgess,  Sc.  D.,2  of  the  Bureau  of  Stand- 
ards, Washington.  The  melting-point  of  aluminium  has  been  added 
by  the  author. 

TABLE  XV.— SCALE  OF  TEMPERATURES 


Temperature 

Possible  error 

Hydrogen  boils 

—  2C2    7°  C 

0    2°  C 

Oxygen  boils  

—  182  9°  C 

o  i°  C 

Mercury  freezes   .... 

—    17   7°  C 

o  i°  C 

Water  freezes  

o  o°  C 

o  o°  C 

Water  boils  

100  o°  C 

o  o°  C 

Tin  melts  

231  85°  C 

o  i°  C 

Cadmium  melts  

?2O    O°  C 

o  i°  C 

Lead  melts 

,27    A°  C 

o  i°  C 

Zinc  melts  

410    4°  C 

o  i°  C 

Sulphur  boils 

AAA     6°    C 

o  i°  C 

Antimony  melts  

630  o°  C 

o  5°  C 

Aluminium  melts 

6<?7  o°  C 

p 

Sodium  chloride  melts  

800  o°  C 

2    0°  C 

Silver  melts  

960  5°  C 

i  o°  C 

Gold  melts 

i  061  o°  C 

2  o°  r 

Copper  melts  

i  083  o°  C 

2    0°  C 

Palladium  melts 

I    <A.Q    0°   C 

10  o°  C 

Platinum  melts  

I  7<r<?    0°  C 

K    0°  C 

Alumina  melts  

2  OOO    O°  C 

30  o°  r 

Tungsten  melts  

3  ooo  o°  C 

100  o°  C 

Surface  of  carbon  arc  

3  600  o°  C 

ISO    0°  C 

Surface  of  sun 

6  ooo  o°  C 

roo   o°  C 

1  W.  C.  Arsem,  The  Electric  Vacuum  Furnace.     Trans.  Amer.  Electrochem. 
Soc.,  vol.  ix,  p.  153. 

2  G.  K.  Burgess,  Met.  and  Chem.  Eng.  x,  1912,  p.  692. 


CHAPTER  VI 
LABORATORY  FURNACES 

In  industrial  operations  the  use  of  the  electric  furnace  is  limited 
by  the  matter  of  cost — electrical  heating  cannot  be  employed 
industrially  for  many  purposes  for  which  it  would  be  suitable, 
because  the  cost  would  be  too  great.  For  experimental  work 
in  the  laboratory,  however,  the  item  of  cost  is  far  less  serious  and 
the  electrical  furnace  has  a  correspondingly  wider  range  of  uses. 

Electric  furnaces  employed  in  the  laboratory  may  be  considered 
under  the  following  heads: 

(1)  Furnaces  suitable  for  making  tests,  such  as  the  determination 
of  the  melting-  and  boiling-points  of  substances,  the  thermal  and 
electrical  conductivity  of  substances,  and  changes  in  volume  and 
other  physical  properties  under  the  influence  of  heat;  such  furnaces 
may  also  be  employed  for  studying  chemical  reactions  such  as  the 
reduction  of  oxides  at  high  temperatures. 

(2)  Furnaces  for  testing  electrical  smelting  processes,  such  as 
the  smelting  of  iron  ores,  the  smelting  of  zinc  ores,  or  the  manufac- 
ture of  steel. 

In  discussing  laboratory  furnaces  it  is  desirable  to  bear  in  mind 
the  essential  differences  between  these  groups  of  furnaces.  Fur- 
naces in  group  (i)  will  be  considered  first;  and  it  will  be  noticed 
that  most  of  them  are  either  crucible  furnaces  or  tube  furnaces. 

TESTING  FURNACES 

For  experimental  purposes  both  arc- furnaces  and  resistance  furnaces 
may  be  used.  The  arc  enables  a  very  high  temperature  to  be  at- 
tained in  a  small  furnace,  but  resistance  furnaces  are  prefer- 
able when  a  uniform  and  easily  regulated  temperature  is  desired. 

Arc -Furnaces 

As  examples  of  these  may  be  mentioned  the  Moissan  furnace, 
Fig.  6,  and  the  Siemens  furnace,  Fig.  2. 

A  convenient  form  of  arc-furnace  for  laboratory  use  is  shown  in 

150 


LABORATORY  FURNACES 


151 


Fig.  63  which  is  modified  from  the  furnace  of  Poulenc  and  Meslans.1 
The  arc  is  direct  heating,  the  carbon  crucible  containing  the  charge 
resting  upon  the  lower  electrode.  The  furnace  is  closed  for  the  re- 
tention of  heat  and  the  exclusion  of  air,  and  can  be  opened  very  easily 
by  means  of  a  lever.  The  furnace  is  lined  with  fire-bricks  and  has 
an  inner,  more  refractory  lining  of  magnesite  or  similar  material. 


W//////////////^^^ 

FIG.  63. — Laboratory  arc  furnace. 

Water-cooled  stuffing-box  electrode  holders  are  employed,  and  the 
furnace  is  supported  on  legs.  The  progress  of  the  operation  can 
be  observed  through  an  opening  provided  with  a  window,  and  a 
neutral  gas  introduced  through  a  pipe  terminated  with  a  stop-cock. 
The  electric  cables  are  led  to  insulated  terminals  on  the  frame  of  the 
furnace,  and  connection  from  these  to  the  electrode  holders  is  made 
by  heavy  copper  pipe  which  also  carries  the  cooling  water. 
1  Borchers'  electric  furnaces,  p.  117. 


152 


THE  ELECTRIC  FURNACE 


LABORATORY  FURNACES 


153 


The  Button  pressure  furnace1  was  employed  by  Dr.  R.  S.  Hutton 
and  Mr.  J.  E.  Petavel  for  the  purpose  of  investigating  the  behavior 
of  substances  at  high  temperatures  and  under  great  pressure.  The 
pressures  to  be  studied  were  as  high  as  200  atmospheres,  and  the  fur- 
nace has  been  very  carefully  designed  to  remain  gas-tight  under  these 
high  pressures.  The  furnace,  which  is  shown  in  Fig.  64,  consists  of  a 
steel  vessel  B  and  lid  D  forming  a  cylindrical  chamber  1 7  in.  long  and 
10  in.  in  diameter  having  hemispherical  ends.  The  cover  D  is  held  in 


Section   A-A 
FIG.  64  A. — Hutton  pressure  furnace. 

position  by  a  number  of  studs  not  shown  in  the  figure.  The  furnace 
has  an  inner  lining  L  of  cast-iron  to  protect  the  steel  shell.  The 
electrodes  are  held  in  metal  holders  which  pass  through  stuffing-boxes 
in  the  ends  of  the  furnace;  the  electrode  holders  being  operated  by 
means  of  a  screw  mechanism,  to  support  them  against  the  high  pressure 
tending  to  force  them  outward.  Openings  in  the  furnace  serve  to 
admit  or  remove  gases,  and  for  observing  the  operation  through 
stout  glass  windows.  The  shell  of  the  furnace  is  surrounded  with  a 

*Dr.  R.  S.  Hutton,  Electrochem.  and  Metall  Ind.,  vol.  vi  (1908),  p.  97;  Phil. 
Trans.  Roy.  Soc.,  series  A,  vol.  ccvii,  1908,  p.  421. 


154  THE  ELECTRIC  FURNACE 

water-jacket  /,  the  cover  has  a  water-jacket  K,  and  the  electrode 
holders  are  also  water-cooled.  The  furnace  can  be  provided  with  a 
refractory  lining  within  the  cast-iron  liner  L,  and  can  be  used  as  an  arc 
or  a  resistance  furnace.  The  electrode  holders  are  hollow  (for  water 
cooling),  and  also  contain  a  small  tube  for  the  introduction  of  a  gas, 
through  a  hollow  electrode,  to  the  center  of  the  furnace.  The  yoke 
F,  which  advances  the  electrode  holder,  is  insulated  from  the  fur- 
nace and  is  connected  to  the  electrical  supply. 

The  furnace  cover  D  and  end  caps  E  make  gas-tight  joints 
with  the  furnace  body  with  the  aid  of  lead  packing- rings.  The  elec- 
trode holder  enters  through  an  insulating  packing,  and  an  insulating 
ring  on  the  holder  screens  this  packing  from  the  heat  of  the  furnace. 
The  furnace  can  be  used  in  the  horizontal  position  or  vertically. 

Resistance  Furnaces 

This  very  important  class  of  laboratory  furnaces  may  be  con- 
sidered for  convenience  under  the  following  heads: 

(i)  Furnaces  with  Metallic  Heating  Coils.— A  furnace  in  this 
class  consists  essentially  of  a  tube  or  crucible  wound  with  a  heating 
coil  of  platinum  or  other  metal.  A  tube  furnace  with  a  platinum 
coil,  shown  diagrammatically  in  Fig.  13,  consists  of  a  tube  of  porce- 
lain, fire-clay,  silica,  or  alundum,  having  a  spiral  heating  coil 
wound  round  it,  and  the  whole  jacketed  to  reduce  the  loss  of  heat. 
These  furnaces  are  very  convenient  for  many  purposes  for  which 
moderate  temperatures  will  suffice.  They  cannot  be  heated  to  the 
melting-point  of  platinum,  and  they  are  also  limited  by  the  fusi- 
bility of  the  material  used  for  the  tube.  A  thermo-couple  pyrom- 
eter can  be  inserted  from  one  end,  and  the  progress  of  the  operation 
can  be  observed  from  the  other  end.  This  form  of  furnace  is  con- 
venient when  it  is  desired  to  have  a  stream  of  gas  flowing  through 
the  furnace.  A  strip  of  platinum  foil  is  sometimes  used  instead 
of  platinum  wire  as,  on  account  of  its  larger  radiating  surface,  a 
greater  rate  of  heating  can  be  obtained  with  the  same  weight  of 
platinum.  Heating  coils  of  nickel,  nichrome  and  other  metals  and 
alloys  are  often  used  on  account  of  their  smaller  cost.  They  cannot 
be  used  at  as  high  a  temperature  as  the  platinum  coils,  and  become 
oxidized  in  time.  Sometimes  a  crucible  is  used  instead  of  a  tube, 
as  in  the  crucible  furnace  designed  by  Prof.  Howe.1  This  furnace, 
Fig  65,  is  made  of  magnesia,  shaped  to  receive  the  crucible  C  and 

1  Howe  Laboratory  Notes,  1902,  p.  37. 


LABORATORY  FURNACES 


155 


having  a  special  groove  to  retain  the  platinum  heating  coil.  A 
thermo-couple  pyrometer  P  is  inserted  through  the  cover,  and  a 
stream  of  gas  can  be  supplied  through  the  pipe  H. 


FIG.  65. — Howe's  crucible  furnace. 

(2)  Furnaces  with  Carbon  Resistors. — 'This  class  includes  tube, 
crucible  and  muffle  furnaces  having  an  external  carbon  resistor  for 
the  production  of  heat.  Furnaces  having  a  carbon  tube  which 
itself  forms  the  resistor  are  considered  in  the  next  class,  as  resistor 
tube, furnaces.  The  following  examples  may  be  given: 

The  Lampen  Tube  Furnace. — This  furnace,  described  by  A. 
Lampen,1  consists  of  a  graphite  tube  T  T,  Fig  66,  heated  by  a  resistor 

1  A.  Lampen,  "An  Electrical  Resistance  Furnace  for  the  Measurement  of  Higher 
Temperatures  with  the  Optical  Pyrometer,"  Jour.  Am.  Chem.  Soc.,  xxviii,  1906, 
p.  846. 


156 


THE  ELECTRIC  FURNACE 


of  broken  carbon,  RR,  which  extends  between  carbon  electrodes  E  E; 
the  electric  current  passing  at  right  angles  to  the  axis  of  the  tube. 
The  tube  is  provided  with  windows,  not  shown  in  the  figure,  to  ex- 
clude the  air  and  to  permit  the  interior  to  be  observed. 

The  resistor  and  the  graphite  tube  are  jacketed  with  some  heat- 
retaining  substance  C  C,  such  as  charcoal  powder,  and  the  ends  of 
the  electrodes  are  surrounded  with  broken  coke  or  carbon.  The 


FIG.  66. — Tucker  and  Lampen  tube  furnace. 

furnace  is  built  of  fire-bricks  and  is  provided  with  a  cover  (not  shown) 
to  prevent  admission  of  air  and  to  reduce  the  loss  of  heat. 

The  furnace  can  be  used  for  measuring  the  temperature  at  which 
substances  melt,  or  at  which  chemical  reactions  take  place;  the  tem- 
perature being  determined  by  means  of  an  optical  pyrometer  sighted 
along  the  tube. 

Fig.  66  actually  represents  a  modification  of  Lampen's  furnace 
employed  by  Tucker  and  Lampen1  for  determining  the  temperatures 

1  S.  A.  Tucker  and  A.  Lampen,  "The  Measurement  of  Temperature  in  the 
Formation  of  Carborundum,"  Jour.  Am.  Chem.  Soc.,  xxviii,  1906,  p.  853;  L.  E. 
Saunders,  Trans.  Am.  Electrochem.  Soc.,  xxi,  1912,  p.  426. 


LABORATORY  FURNACES 


157 


of  formation  and  decomposition  of  carborundum.  The  filling  C 
being  the  mixture  of  sand  and  carbon  which  is  converted  into  car- 
borundum by  heat  produced  in  the  resistor  R  R.  A  piece  of  graphite 
is  placed  in  the  tube  T  T,  and  by  observing  this  with  an  optical  py- 
rometer, the  temperatures  at  different  parts  of  the  furnace  can  be 
ascertained. 

Crucible  Furnaces. — One  of  these  is  shown  in  Fig.  14,  and  an- 
other which  was  used  in  the  author's  laboratory  for  testing  the 
electrical  resistivity  of  fire-bricks  is  shown  in  Fig.  34.  In  the  latter 
the  crucible  was  of  graphite  and  clay,  and  temperatures  up  to 
1 600°  C.  could  conveniently  be  obtained.  In  furnaces  of  this  class, 
kryptol  (see  page  291)  is  often  used  instead  of  carbon  or  graphite 
for  the  resistor.  A  furnace  having  ring-shaped  electrodes  is  shown 
in  Fig.  67.  The  ring-shaped  electrodes  cause  a  more  even  heating 
of  the  crucible  than  would  be  obtained  in  the  other  furnaces. 


FIG.  67.— Crucible  furnace. 

In  all  these  furnaces  the  temperature  that  can  be  obtained  depends 
upon  the  melting-point  of  the  crucible  or  tube.  Carbon  itself  is 
quite  infusible,  but  sometimes  carbon  is  inadmissible  and  then  the 
tube  or  crucible  may  be  made  of  carborundum  for  very  high  tempera- 
tures, while  quartz,  alundum  or  fire-clay  may  be  used  for  lower  tem- 
peratures. It  should  be  remembered  that  the  oxides  forming  the 
refractory  envelope  are  liable  to  be  reduced  to  metals  by  the  carbon 
resistor  at  high  temperatures 

In  the  figure,  R  R  are  the  ring-shaped  electrodes  which  are  sup- 
plied with  current  from  the  water-cooled  electrode  holders  H  H.  The 
granular  carbon,  which  forms  the  resistor,  is  shown  at  CC  surround- 


158  THE  ELECTRIC  FURNACE 

ing  the  crucible.  The  furnace  is  constructed  of  fire-clay  within  an 
iron  casing,  but  a  lining  LLof  particularly  refractory  material,  such 
as  magnesia  or  carborundum,  is  placed  between  the  carbon  rings 
and  surrounds  the  resistor.  The  furnace  is  designed  so  that  the 
broken  carbon  can  be  filled  in  around  the  crucible.  The  crucible 
itself  should  be  a  non-conductor,  as  otherwise  it  will  carry  the 
current,  thus  short-circuiting  part  of  the  resistor. 

Arsem  Vacuum-furnace. — A  very  carefully  designed  furnace  for 
operating  in  a  vacuum  was  described  by  W.  C.  Arsem1  in  the  year 
1906,  and  is  shown  in  Fig.  68.2  A  gun-metal  chamber,  A,  which  can 
be  rendered  vacuous  by  means  of  a  pump,  contains  the  furnace 
proper.  This  consists  of  a  heater  L  which  is  a  spiral  of  graphite, 
within  which  a  crucible  Y  can  be  supported;  a  radiation  screen  0, 
made  of  graphite  and  filled  with  powdered  graphite,  serves  to 
minimize  the  loss  of  heat  by  radiation.  Electrical  connection  is 
made  to  the  ends  of  the  graphite  spiral  by  means  of  copper 
clamps  U  which  are  water-cooled;  the  pipes  conveying  the  cooling 
water  serve  also  to  carry  the  electric  current  to  the  clamps.  A 
window  E  of  mica  enables  the  operation  to  be  observed.  Con- 
sidering the  construction  of  this  furnace  it  may  appear  at 
first  that  the  radiation  screen  O  might  have  been  better  designed 
for  retaining  the  heat,  but  it  should  be  remembered  that  when 
operating  in  a  vacuum  there  is  not  the  loss  of  heat  which  occurs  in 
ordinary  furnaces  due  to  the  circulation  of  heated  air;  moreover, 
the  small  mass  of  the  radiation  screen  allows  the  furnace  to  arrive 
very  rapidly  at  its  final  temperature,  and  this  enables  the  tempera- 
ture of  the  furnace  to  be  determined  quite  accurately  from  an 
observation  of  the  power  supply.  The  furnace  is  calibrated  by 
melting  in  it  metals  of  varying  fusibility  and  the  current  required 
to  melt  each  metal  is  ascertained.  A  calibration  curve  can  then  be 
drawn  giving  the  relation  between  current  and  temperature,  and  this 
curve  is  of  such  a  nature  that  temperatures  up  to  nearly  3,000°  C. 
can  be  determined  with  a  probable  error  of  only  50°  C.  The  fur- 
nace can  be  used  for  a  considerable  time  at  temperatures  up  to  about 
2,000°  C.,  but  at  higher  temperatures  the  graphite  of  the  spiral 
volatilizes  and  wastes  away  until  a  break  occurs.  The  furnace  was 
found  to  run  for  about  nine  hours  at  2,500°  C.,  and  for  only  one  hour 
at  3,000°  C.  The  whole  furnace  is  placed  in  a  metal  chamber  R 

1W.  C.  Arsem,  "The  Electric  Vacuum  Furnace,"  Am.  Electrochem.  Soc., 
ix  (1906),  p.  153. 

2  Reproduced  from  Mr.  Arsem's  paper. 


LABORATORY  FURNACES 


159 


which  is  filled  with  water  for  the  purpose  of  keeping  the  vessel  A 
from  becoming  heated. 

The  Arsem  vacuum  furnace  is  made  in  several   forms   by    the 
General  Electric  Company,  Schenectady.1     The  form  described  is 


FIG.  68. — Arsem  vacuum  furnace. 


21  in.  high  and  15  in.  in  diameter,  and  contains  a  crucible  1.5  in.  in 
diameter  and  4  in.  high.  It  takes  15  kw.  (250  amperes  at  60  volts), 
and  attains  a  temperature  of  3,100°  C.  A  larger  furnace  is  made 
holding  a  crucible  10  in.  high  and  4  in.  diameter,  and  using  60  kw. 
A  tube  furnace  and  box-type  furnace  are  also  made. 

1  General  Electric  Company,  Bull.  4898  A,  April,  1912. 


160 


THE  ELECTRIC  FURNACE 


(3)  Resistor  Tube  Furnaces. — A  furnace  in  this  class  consists  of  a 
conducting  tube,  suitably  jacketed,  with  provision  for  passing  an 
electric  current  along  the  tube  to  heat  it.  The  tube  is  usually 
made  of  carbon  or  graphite,  but  sometimes  other  conducting 
materials  are  used,  such  as  the  Nernst  earths  which  become 
conductors  when  heated.  A  furnace  having  a  tube  of  graphite  or 
carbon  has  been  described  by  Hutton.1 

Fig.  69  shows  a  resistor  tube  furnace  similar  in  principle  to  that  of 


FIG.  69. — Resistor  tube  furnace. 

Hutton.  It  consists  of  an  iron  case  supported  on  legs  and  lined 
with  fire-bricks.  The  carbon  tube  passing  through  the  furnace  has 
its  ends  coppered  and  is  soft-soldered  into  water-cooled  copper 
holders.  The  holders  are  provided  with  windows,  and  a  neutral 
gas,  such  as  hydrogen,  can  be  passed  through  the  carbon  tube. 
Electrical  connection  is  made  from  the  holders  by  flexible  cables  to 
insulated  terminals  on  the  framework  of  the  furnace.  A  large  current 
1  R.  S.  Hutton  and  W.  H.  Patterson,  Trans.  Faraday  Soc.,  i,  1905,  p.  187; 
Electrochem.  and  Metall.  Ind.,  iii,  1905,  p.  455. 


LABORATORY  FURNACES 


161 


at  a  low  voltage  will  usually  be  needed  for  heating  the  tube,  and  if 
necessary  the  middle  part  of  the  tube  can  be  made  somewhat  thinner 
so  as  to  increase  its  electrical  resistance.  The  tube  is  jacketed  and 
.  protected  from  oxidation  by  a  powdery  material  such  as  amorphous 
carborundum  which  fills  the  body  of  the  furnace. 

Harker  Laboratory  Furnace.1 — This  furnace,  Fig.  70,  consists  of  a 
tube,  T,  composed  of  earths  like  those  used  in  the  Nernst  filament; 
these  earths  are  non-conducting  when  cold  but  become  conductors 
when  heated  to  a  red  heat.  The  current  is  supplied  to  the  ends  of 
this  tube  by  means  of  platinum  conductors,  P  P,  and  the  tube  itself 


FIG.  70.     Harker  tube  furnace. 

is  jacketed  with  zirconia  powder,  Z.  An  outer  heating  coil  of  nickel, 
N  N,  is  used  to  heat  the  tube  to  the  temperature  at  which  it  becomes 
conducting,  and  also  to  supply  auxiliary  heat;  thus  enabling  the 
tube  to  be  used  with  a  smaller  current.  The  nickel  coil  is  wound 
around  a  fire-clay  tube,  and  the  whole  is  suitably  jacketed.  This 
furnace  can  be  used  up  to  about  2,200°  C.,  and  as  this  temperature 
is  above  the  melting-point  of  the  platinum  conductors  it  should  be 
noted  that  only  the  part  of  the  tube  remote  from  the  conductors  will 
attain  this  temperature. 

1  J.  A.  Harker,  Electrochem.  and  Metall.  Ind.,  vol.  iii  (1905),  p.  273. 
11 


162 


THE  ELECTRIC  FURNACE 


SMELTING  FURNACES 

The  furnaces  already  described  for  carrying  out  certain  tests  and 
other  regular  laboratory  operations  tend  to  conform  to  certain 
definite  types  and  one  may  hope  that  they  will  become  standardized, 
but  for  testing  electric  smelting  processes  it  is  necessary  that  the 
laboratory  furnace  should  follow  somewhat  closely  the  design  of  the 
commercial  furnace  whose  operations  are  to  be  imitated.  Examples 
will  be  given  of  small-scale  furnaces  for  the  production  of  pig-iron, 
steel,  zinc,  etc. 

Heroult  Steel  Furnace. — A  small  furnace  of  this  type  constructed 
by  C.  A.  Hansen1  is  shown  diagrammatically  in  Fig.  71.  It  consists 


FIG.  71. — Hansen  steel  furnace. 

of  an  iron  box  mounted  on  trunnions,  and  provided  with  mechanism 
for  tilting.  The  furnace  is  lined  with  4  1/2  in.  of  magnesite 
bricks,  for  the  walls,  and  with  about  8  in.  of  brickwork  and  some 
rammed  magnesite  for  the  bottom.  The  roof  is  an  arch  of  silica 
brick.  The  electrodes  are  of  graphite,  and  are  supported  by  holders 
having  a  screw  adjustment. 

The  power  used  was  about  80  kw.  (1000  amperes  at  84  volts) 
and  the  charge  of  steel  melted  varied  from  50  Ib.  to  300  Ib.  A 
charge  of  1 50  Ib.  of  sheet  scrap  can  be  melted  in  a  cold  furnace  with 
about  70  kw.-hours.  The  furnace  can  be  kept  hot  with  50  kw.  and 

1  C.  A.  Hansen,  "Small  Experimental  Heroult  Furnace,"  Electrochem.  and 
Met.  Ind.,  vii,  1909,  p.  206. 


LABORATORY  FURNACES 


163 


with  300  Ib.  charges  the  power  consumption  averaged  about  150  kw.- 
hours  per  heat,  or  1,000  kw.-hours  per  ton. 

A  tilting  arc-furnace  of  the  Heroult  type,  which  has  been  built 
in  the  author's  laboratory,  is  shown  in  Fig.  72.  This  has  been  built, 
for  greater  flexibility  of  use,  with  the  vertical  electrodes  of  the  Her- 
oult furnace  and  also  with  the  nearly  horizontal  electrodes  of  the 
Stassano  furnace.  It  consists  of  a  steel  plate  box  on  rockers,  resting 
on  a  carriage,  so  that  the  whole  furnace  can  be  moved  about,  and 
is  provided  with  a  spout,  working  door  and  electrode  holders. 
The  electrode  holders  are  water-cooled  brass  stuffing-boxes  with 
metallic  packing,  which  admit  the  graphite  electrodes  and  make 
electrical  contact  with  them  as  well  as  cooling  them  and  preserving 


FIG.  72. — Steel  furnace  at  McGill. 

a  gas-tight  joint.  The  electrical  supply  is  led  to  the  pair  of  elec- 
trode holders  at  each  end  of  the  furnace  by  means  of  a  heavy  copper 
pipe  which  also  supplies  the  cooling  water.  Rubber  connections 
for  the  water  are  made  between  the  holders  at  one  end  and  those 
at  the  other  end  of  the  furnace. 

The  lining  of  the  furnace  has  been  very  carefully  designed  to 
retain  the  heat  as  far  as  possible.  Beginning  with  a  lining  of  sheet 
asbestos,  the  bottom  of  the  furnace  has  a  few  rows  of  bricks  on  edge; 
the  spaces  between  the  rows  being  filled  with  kieselguhr.  On 
the  top  of  these  bricks  is  a  course  of  bricks  laid  flat  and  a  rammed 
lining  of  burnt  magnesite  and  tar  comes  above  this.  The  sides  of 
the  furnace  are  lined  with  a  4-S-in.  course  of  fire-bricks,  with 


164  THE  ELECTRIC  FURNACE 

kieselguhr  between  this  and  the  asbestos  sheet,  and  a  rammed 
lining  within  the  fire-bricks.  The  roof  is  arched  and  constructed 
of  silica  bricks  with  an  outer  covering  of  sheet  asbestos.  The  roof 
is  constructed  in  a  steel  frame  and  can  be  lifted  from  the  furnace 
by  means  of  a  chain  and  blocks.  This  furnace  will  hold  100  Ib. 
of  molten  steel,  and  is  usually  run  with  about  250  amperes  at  100 
volts.  * 

The  drawing  shows  the  original  design  which  has  since  been  modi- 
fied. The  electrodes,  while  actually  stout  enough  to  carry  the 
250  amperes  which  is  the  normal  supply  at  100  volts,  have  now  been 
made  heavier  and  are  usually  supplied  with  300  amperes  at  80  or 
90  volts.  The  furnace  was  found  to  be  somewhat  too  large,  even 
with  the  very  careful  jacketing,  for  the  small  amount  of  power 
available,  and  has  now  been  reduced  in  size  by  increasing  the 
thickness  of  the  lining  and  setting  the  electrodes  closer  together. 
Even  so  the  furnace  is  somewhat  short  of  power  and  cannot  produce 
the  results  obtained  in  the  Hansen  furnace  using  80  kw.  This  fur- 
nace has  generally  been  used  in  the  production  of  steel  from  ore  and 
the  results  are  therefore  not  directly  comparable  with  those  obtained 
by  Mr.  Hansen. 

Another  furnace  for  making  steel  in  use  in  the  author's  laboratory 
is  a  2o-kw.  Colby  induction  furnace1,  Fig.  73,  which  has  done  satis- 
factory work.  The  primary  winding  of  this  furnace  consists  of  24 
turns  of  hydraulic  copper  pipe  which  is  water-cooled.  It  was 
built  for  6o-cycle  current  but  works  satisfactorily  at  25  cycles. 

The  furnace  holds  about  40  Ib.  of  steel,  and  takes  200  or  250 
amperes  at  no  volts.  With  25-cycle  current  the  power-factor  is 
high,  being  over  90  per  cent,  when  the  furnace  is  full.  The  efficiency 
is  low,  however,  there  being  a  loss  of  about  6  kw.  in  the  cooling 
water,  and  the  iron  core  becomes  very  hot. 

The  furnace  consists  of  a  laminated  iron  core  mounted  on  a  base- 
plate and  having  a  primary  winding  of  copper  pipe  shown  by  circles 
in  the  elevation.  Around  this  is  the  annular  crucible,  made  of  a 
graphite-clay  mixture  and  containing  the  melted  steel.  The  crucible 
stands  on  a  soapstone  base  and  is  lagged  with  kieselguhr.  The 
whole  is  supported  on  trunnions,  and  can  be  tilted  to  pour  the  steel, 
by  means  of  gearing.  A  four-wheeled  carriage,  added  in  the  McGill 
laboratories,  enables  the  furnace  to  be  easily  moved. 

The  furnace  is  started  by  means  of  a  cast-iron  ring,   which  is 

1  Supplied  by  the  American  Electric  Furnace  Co.  See  description  of  the 
Colby  furnace  in  Chapter  VIII. 


LA  BORA  TOR  Y  F  URN  A  CES 


165 


FIG.  73. — Colby  induction  furnace. 


166 


THE  ELECTRIC  FURNACE 


placed  in  the  crucible  (the  upper  limb  of  the  core  being  removable 
for  this  purpose)  and  when  this  has  melted,  further  additions  of 
iron  or  steel  can  be  made  as  desired. 

Pig-iron  Furnace. — The  reduction  of  iron-ore  to  pig-iron  is  easily 
demonstrated  in  a  furnace  built  of  fire-bricks  about  9  in.  square 
inside  with  a  4-5-in.  wall,  as  shown  in  Fig.  74.  A  graphite  elec- 
trode, A ,  is  built  into  the  furnace,  and  a  lining  of  coke  and  tar  is  ram- 
med around  this  and  continued  upward  to  form  a  working  lining  for 
the  lower  part  of  the  furnace.  The  upper  conductor  consists  of  a 
graphite  electrode,  B,  supported  by  the  holder  shown  in  Fig.  75.  The 


FIG.  74. — Iron-smelting  furnace. 


furnace  is  dried  out  and  the  lining  baked  by  passing  a  moderate 
current  for  some  hours  through  a  little  broken  coke  in  the  furnace. 
The  charge,  consisting  of  ore,  charcoal  and  limestone  is  fed  in  around 
the  upper  electrode,  and  the  current  gradually  increased  until  the 
furnace  is  in  regular  operation;  400  or  500  amperes  at  50  volts 
are  suitable  for  a  furnace  of  this  size,  and  a  charge  of  20  Ib.  of  ore 
can  be  smelted  in  about  an  hour.  When  enough  ore  has  been 
charged  the  smelting  is  continued  until  the  contents  of  the  furnace 
are  melted,  and  the  molten  metal  and  slag  are  tapped  out  through 
the  tapping  hole. 

A  furnace  like  the  one  just  described  can  be  used  for  the  produc- 


LABORATORY  FURNACES 


167 


tion  of  ferro-silicon  and  similar  products.  For  making  calcium 
carbide,  alundum,  or  silicon,  a  simple  pit  furnace  can  be  used,  with 
two  or  more  vertical  electrodes  as  shown  in  Fig.  75.  The  furnace 


Scale  of  Inches 

FIG.  75. — Adjustable  electrode-holder  and  furnace. 

is  built  of  fire-brick  and  needs  no  rammed  lining,  though  one  is 
shown  in  the  figure,  as  the  materials  of  the  charge  form  the  working 
lining. 


168 


THE  ELECTRIC  FURNACE 


For  furnaces  of  the  type  in  which  calcium  carbide  or  ferro-alloys 
are  made,  a  special  electrode  holder,  Fig.  75,  has  been  constructed 
which  is  very  generally  useful.  This  consists  of  a  tall  steel  frame 
on  which  slides  vertically  a  pair  of  blocks  connected  together  by  a 
long  screw,  one  of  these  blocks  can  be  clamped  to  the  standards,  and 
the  other  block,  which  carries  a  pair'  of  arms,  moves  up  and  down 
by  means  of  the  screw,  the  arms  carry  at  one  end  terminals  for  the 
electric  cables  of  the  supply  and  at  the  other  end  gun-metal  electrode 
holders  into  each  of  which  are  threaded  either  one  or  two  electrodes. 
The  electrode  holders  are  water-cooled;  the  water  for  this  purpose 
passing  in  and  out  through  hydraulic  copper  pipes,  which  also  help 
to  carry  the  electric  current  from  the  cables  to  the  electrode  holders. 


FIG.  76. — Silicon  furnace. 


The  furnaces  for  use  with  this  apparatus  are  usually  built  up  as 
required  from  a  few  fire-bricks,  the  lining  being  a  part  of  the  charge 
itself  as  in  the  carbide  furnace,  or  a  rammed  lining  of  magnesite  and 
tar,  or  coke  and  tar,  may  be  employed.  The  production  of  pig- 
iron  from  the  ore  is  carried  out  with  this  apparatus. 

Silicon  can  be  made  in  the  laboratory  as  has  been  shown  by 
Prof.  S.  A.  Tucker,1  by  means  of  an  electric  arc  in  the  midst  of  a 
charge  of  sea-sand  and  coke  well  mixed  and  crushed  to  pass  through 
a  ten- mesh  sieve. 

A  rectangular  brick  furnace  10  in.  X  9  in.  X  8  in.  high  inside  is 
used  with  a  pair  of  2-in.  graphite  electrodes  as  shown  in  Fig.  76. 

1  S.  A.  Tucker,  "The  Preparation  of  Silicon  in  the  Laboratory,"  Met.  and 
Chem.  Eng.,  viii,  1910,  p.  19. 


LABORATORY  FURNACES  169 

Silicon  is  reduced  from  silica  by  carbon  according  to  the  reaction: 


2  =  28+2(i2+i6)     1  Parts  by 
60+24  =  28+56  j      weight. 

This  shows  that  5  parts  of  silica  would  need  2  parts  of  carbon 
for  its  reduction.  Prof.  Tucker  employs  an  excess  of  silica,  using 
77  parts  of  silica  with  25  parts  of  coke.  This  excess  of  silica  is  in- 
tended to  combine  with  the  carbon  from  the  electrodes,  which 
inevitably  takes  part  in  the  reaction,  so  that  the  silicon  shall  not  be 
carbonized. 

For  a  furnace  of  the  size  indicated  he  uses  a  charge  of  12  kilos 
(26  Ib.)  of  the  mixture,  and  a  current  of  about  500  amperes  at  50 
volts  for  one  and  a  quarter  hours.  Only  about  one-quarter  of  the 
charge  is  reduced,  forming  about  200  grm.  (7  oz.)  of  silicon;  the  re- 
mainder of  the  charge  serving  as  the  working  lining  and  cover  of 
the  furnace.  The  silicon  runs  down  through  the  charge  to  the  bot- 
tom of  the  furnace  and  thus  escapes  from  the  region  of  the  arc. 
This  is  essential  as  otherwise  it  would  volatilize  and  be  lost. 

Silicon  can  also  be  made  in  a  pit  furnace  using  two  vertical 
depending  electrodes.  The  charge  may  be  made  up  of  sand  or 
crushed  quartz,  briquetted  with  pitch  and  coal  or  coke,  and  used 
in  coarse  grains  which  allow  the  gas  to  escape  more  easily. 

The  Acheson  carborundum  and  graphite  furnaces  can  be  built 
on  a  small  scale  using  a  pair  of  horizontal  2-in.  graphite  electrodes 
in  the  holders  shown  in  Fig.  45.  The  electrode  holders  for  this 
furnace  are,  at  each  end,  a  pair  of  water-cooled  half  collars  of  brass 
which  are  supported  and  clamped  together  by  two  threaded  stand- 
ards. In  operating  this  furnace,  information  can  be  gained  as  to 
the  amount  of  heat  carried  to  the  electrode  holders,  as  this  is  all 
removed  in  the  cooling  water  and  can  be  measured.  For  these 
operations  a  large  current  at  a  low  voltage  is  suitable,  1,000  or  1,200 
amperes  at  25  volts  being  sometimes  used  in  a  small  furnace.  Car- 
borundum has  been  made  in  this  way,  using  a  core  of  broken  car- 
bon having  grains  between  1/4  in.  and  1/8  in.  in  size.  The  core 
was  4  in.  square  in  cross-  section,  and  the  electrodes  were  6  in. 
apart;  their  ends  being  surrounded  by  the  core.  A  constant  power 
of  15  kw.  was  supplied  during  12  hours;  the  voltage  being  55  to 
begin  with  and  dropping  to  30  when  the  furnace  was  thoroughly 
hot.  The  product  included  about  2  Ib.  of  crystallized  carborundum 
and  a  small  amount  of  graphite,  resulting  from  the  decomposition 


170  THE  ELECTRIC  FURNACE 

of  the  inner  layer  of  carborundum.  The  author  has  devoted  a 
great  deal  of  time  to  experiments  on  the  production  of  zinc  from 
its  ore  in  small  electric  furnaces;  some  of  which  are  described  in 
Chapter  XII. 

All  the  furnaces  described  in  this  chapter  are  operated  by  alter- 
nating current.  Operations  involving  electrolysis  such  as  the  for- 
mation of  sodium  or  aluminium  require  direct  current  and  are  con- 
sidered in  Chapter  XIV. 

Power  for  Electric  Furnaces  at  McGill. — Since  coming  to  McGill 
University  in  1901,  the  author  has  devoted  considerable  attention 
to  electric-furnace  work  and  has  made  many  experiments  with  elec- 
tric furnaces  in  the  laboratory.  Until  recently,  the  power  for  this 
purpose  was  taken  from  the  no-volt  direct- current  supply;  not  more 
than  200  amperes  could  be  used,  and  whenever  furnaces  were  run 
at  low  voltages  the  use  of  this  power  was  very  wasteful.  In  1909 
a  new  power-house  was  erected,  supplying  direct  current  at  220 
volts  for  power  throughout  the  University,  and  this  would  have 
been  even  less  suitable  for  direct  use  in  electric  furnaces. 

Dr.  Milton  Hersey  of  Montreal,  an  old  McGill  graduate,  gener- 
ously subscribed  a  considerable  sum  of  money  to  the  Metallurgical 
Department,  a  part  of  which  has  been  devoted  to  the  purchase  of  a 
motor-generator  set,  transformer  and  electrical  measuring  instru- 
ments for  electric  smelting.  It  was  originally  intended  to  have 
alternating  and  direct  current  at  variable  voltages,  but  it  was  found 
to  be  difficult  to  obtain  both  from  the  same  generator  and  it  was 
decided  to  provide  for  alternating  current  alone  in  the  first  instance. 

The  installation  (see  Fig.  77)  consists  of  a  5o-h.p.  220- volt  direct- 
current  motor  having  a  3o-kw.  2 20- volt  alternating- current  generator 
on  the  same  shaft.  The  generator  supplies  the  high-tension  wind- 
ings of  a  transformer,  the  secondary  low- tension  windings  of  which 
are  in  four  separate  parts.  The  terminals  from  the  secondary 
windings  are  brought  to  a  set  of  mercury  cups,  by  means  of  which 
they  can  easily  be  grouped  in  series,  parallel  or  multiple  series; 
thus  obtaining  the  current  at  voltages  of  no,  55  and  27.5  volts. 
The  field  of  the  generator  has  a  regulating  rheostat  by  means  of 
which  the  generator  voltage  can  be  altered  within  wide  limits,  so 
that  one  has  a  perfect  control  of  the  voltage  supplied  to  the  furnace. 
The  generator  is  three-phase,  but  until  recently  only  one  phase 
was  used  and  this  supplied  the  full  30  kw.  at  which  the  generator  is 
rated.  An  additional  transformer  has  now  been  provided  for  sup- 
plying furnaces  with  two-phase  or  three-phase  current.  The  gener- 


LABORATORY  FURNACES 


171 


at  or  has  been  built  for  25  cycles  as  this  is  more  generally  suitable  for 
electric  furnaces  than  a  higher  frequency. 

A  complete  set  of  measuring  instruments  has  been  provided 
enabling  the  current,  voltage  and  power  supplied  to  any  furnace 
to  be  easily  and  accurately  measured.  The  amount  of  current 


FIG.  77. — Furnace  connections  in  McGill  laboratory. 

available  from  the  transformer  is  250  amperes  at  no  volts,  500 
amperes  at  55  volts  or  1,000  amperes  at  27  1/2  volts,  but  decidedly 
larger  currents  can  be  used  for  short  periods  without  endangering 
the  apparatus.  The  motor-generator,  both  on  account  of  its  inertia 
and  of  its  "regulation"  causes  the  draft  of  power  from  the  supply 
to  be  far  less  irregular  than  if  the  furnaces  were  connected  directly 


172  THE  ELECTRIC  FURNACE 

to  the  line.  The  line  supplying  the  motor-generator  is  protected 
by  a  magnetic  circuit  breaker  which  is  usually  set  for  250  amperes. 
A  simple  automatic  regulator  is  also  provided,  which  consists  of  a 
rheostat  and  a  magnetic  circuit  breaker.  The  breaker  is  actuated 
by  the  current  supplying  the  motor  but  instead  of  breaking  this 
circuit  it  puts  an  additional  resistance  in  series  with  the  generator 
field;  thus  lowering  the  voltage  of  the  generator.  This  breaker  is 
arranged  to  come  into  action  when  the  main  current  exceeds  about 
200  amperes.  When  this  occurs  the  field  rheostat  is  adjusted  by 
hand  and  the  breaker  closed  again. 

The  installation  is  shown  diagrammatically  in  Fig.  77.  M  is  the 
5o-hp.  motor,  connected  to  the  2 20- volt  direct- current  supply,  with 
the  field  rheostat  R\  which  serves  to  control  the  speed.  The  start- 
ing switches  and  breakers  are  not  shown  in  the  diagram.  G  is  the 
three-phase  alternating- current  generator  on  the  same  shaft  as  the 
motor.  The  revolving  field  is  supplied  with  current  from  the  220- 
volt  direct-current  line  through  the  rheostat  R%  which  serves  to  regu- 
late the  voltage  of  the  generator.  The  high-tension  winding  of  the 
transformer  T  is  connected  to  two  of  the  leads  i,  2,  3,  from  the  gen- 
erator. The  eight  terminals  of  the  secondary  windings  are  bolted 
to  mercury  cups  on  the  table  E,  and  these  are  coupled  as  required 
to  the  two  omnibus-bars  A  B  and  C  D.  The  bar  AB  has  terminals 
beneath  it  for  bolting  on  the  return  cables  from  the  furnaces.  Con- 
nections from  C  D  pass  through  the  current  transformers  C  T\  and 
C  TI  to  a  pair  of  terminal  bars  from  which  cables  are  carried  to  the 
furnaces.  C  TI  has  a  ratio  of  600  to  5  and  serves  for  currents  under 
600  amperes,  while  C  Tz  has  a  ratio  of  1600  to  5  and  is  used  for  larger 
currents.  In  general  one  furnace  only  is  in  use  at  any  one  time,  but 
two  furnaces  can  be  operated,  at  the  same  voltage,  and  the  current 
taken  by  each  measured  with  the  aid  of  the  two  current  transformers. 
The  omnibus- bar  C  D  is  in  two  parts,  and  when  these  are  separated 
by  removing  the  connecting  link,  two  furnaces,  FI  and  F2  can  be 
supplied  with  current  at  different  voltages. 

G  H  is  the  instrument  board  with  voltage  connections  v  v  v  from 
the  various  points  and  current  connections  from  the  current  trans- 
formers through  the  switch  S  to  the  ammeter  A  2  and  wattmeter  W ' . 

The  voltmeters  Vi,  ¥2,  having  ranges  of  75  volts  and  150  volts, 
and  the  voltage  terminals  of  the  wattmeter  are  connected  by  flexi- 
ble cords  and  plugs. 


CHAPTER  VII 
THE  PRODUCTION  OF  PIG-IRON  IN  THE  ELECTRIC  FURNACE 

Iron  is  employed  in  the  mechanic  arts  in  combination  with  vari- 
able amounts  of  carbon  and  other  metalloids  and  metals,  as 
wrought  iron,  cast  iron,  or  steel.  These  terms  cover  a  wide  range 
of  different  materials.1 

Cast-iron,  or  pig-iron,  is  the  form  in  which  the  metal  is  usually 
obtained  from  the  ore;  it  contains  from  2  per  cent,  to  4.5  per  cent, 
of  carbon,  from  1/2  per  cent,  to  4  per  cent,  of  silicon,  and  small  but 
variable  amounts  of  manganese,  sulphur  and  phosphorus;  the  re- 
mainder being  iron.  The  carbon  and  other  elements  are  absorbed 
by  the  iron  during  its  production  in  the  blast-furnace,  and  make  it 
more  easily  fusible  than  if  it  were  pure;  the  melting  temperature 
of  pure  iron  being  1505°  C.,  or  2740°  F.,  while  that  of  cast-iron 
varies  from  about  1027°  C.  to  1275°  C.,  or  from  1880°  F.  to  2327°  F., 
depending  upon  its  composition.  The  fusibility  of  cast-iron  makes 
it  suitable  for  use  in  the  foundry,  but  the  presence  of  a  large  amount 
of  carbon  and  other  metalloids  renders  it  far  less  valuable  mechanic- 
ally than  the  purer  forms  of  wrought-iron  and  steel. 

Wrought-iron  consists  of  nearly  pure  iron,  retaining  only  small 
amounts  of  carbon  and  other  metalloids,  together  with  a  small 
amount  of  admixed  slag.  It  is  made  by  melting  pig-iron  in  the, 
"  puddling  "  furnace  in  contact  with  a  cinder  or  slag  rich  in  oxides  of 
iron.  The  carbon  and  other  metalloids  in  the  pig-iron  are  largely 
removed  by  reaction  with  this  slag  and  the  nearly  pure  iron  forms 
in  grains  in  the  furnace,  being  too  infusible  to  be  melted.  These 
grains  of  iron  are  welded  together,  but  still  retain  some  of  the  slag 
from  the  furnace.  The  puddled  iron,  after  being  rolled  into  bars, 
is  cut  into  short  pieces  which  are  made  into  bundles  or  "piles," 
which  are  reheated  and  rolled  into  bars  or  other  shapes.  The 
operation  of  "piling"  removes  some  of  the  slag,  and  improves  the 
quality  of  the  iron.  A  large  amount  of  so-called  wrought-iron  is 
made  by  piling  pieces  of  mild  steel. 

1  The  different  varieties  of  iron  and  steel  have  been  defined  by  the  International 
Association  for  Testing  Materials.  Jour.  Iron  and  Steel  Inst.,  1906,  iv,  p.  699. 

173 


174  THE  ELECTRIC  FURNACE 

Steel  is  a  very  comprehensive  term,  and  includes: 

(a)  Crucible  steel,  which  is  made  from  carefully  selected  varie- 
ties of  wrought-iron  or  steel,  has  been  melted  in  crucibles,  and  con- 
tains from  about  3/4  per  cent,  to  1.5  per  cent,  of  carbon,  together 
with  enough  manganese  and  silicon  to  produce  a  sound  casting. 

(b)  Bessemer  and  open- hearth  steels  include  all  the  products  of 
these  furnaces,  and  may  range  from  the  hardest  of  tool  steel  to  a 
material  which  is  practically  pure  iron,  and  only  differs  from  wrought- 
iron  in  having  been  fused,  and  being  in  consequence  nearly  free 
from  slag,  and  in  the  presence  of  a  little  manganese,  added  to  ensure  a 
sound  casting. 

The  production  of  iron  and  steel  in  the  electric  furnace  may  be 
considered  under  three  heads: 

I.  The  production  of  pig-iron  by  heating  iron-ore  with  carbon  and 
fluxes  in  an  electric  furnace. 

II.  The  production  of  steel  by  melting  steel  scrap,  either  alone 
or  with  the  addition  of  pig-iron,  iron-ore,  etc.,  in  an  electric  furnace. 

III.  The  production  of  steel  by  heating  iron-ore  with  carbon  and 
fluxes  in  an  electric  furnace. 


THE  PRODUCTION  OF  PIG-IRON 

Pig-iron  is  generally  produced  by  smelting  iron-ore  in  the  blast- 
furnace with  carbonaceous  fuel,  usually  coke  or  charcoal,  and 
limestone. 

The  ore  is  almost  always  an  oxide  of  iron  and  the  coke  or  charcoal 
serves  the  double  purpose  of  supplying  heat  by  its  combustion  in 
the  blast  of  air  and  of  "reducing"  the  oxide  of  iron  to  metal  by 
combining  with  its  oxygen.  The  fuel  does  not  burn  completely  in 
the  blast,  forming  carbon  dioxide,  but  only  incompletely  with  the 
formation  of  carbon  monoxide.  It  is  the  carbon  monoxide  so  pro- 
duced that  is  mainly  effective  in  reducing  the  oxide  of  iron  to  metal. 
The  metallic  iron  becomes  saturated  with  carbon,  forming  pig-iron 
which  accumulates  in  the  bottom  of  the  furnace  and  is  tapped  out  at 
intervals. 

After  the  oxide  of  iron  has  been  reduced  to  metal  there  remains 
the  gangue  of  the  ore  which  is  mostly  silicious  and  clayey  in  character, 
and  the  ash  from  the  coke.  This  is  fluxed,  or  rendered  fusible,  by 
the  limestone,  which  was  included  in  the  charge,  and  melts  down 
forming  the  slag. 

It  should  be  noted  that  the  iron  oxide  is  reduced  to  metal,  whereas 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  175 

the  lime,  silica,  and  alumina  in  the  charge,  which  are  also  oxides, 
are  not  reduced  to  the  metallic  state  but  remain  as  oxides  and  so 
form  a  slag.  This  is  because  iron  oxide  is  reduced  more  easily  than 
the  other  oxides  named.  If  the  temperature  were  high  enough,  all 
these  oxides  would  be  reduced  and  the  metallic  base  of  each  would 
enter  the  pig-iron. 

In  the  blast-furnace  a  small  part  of  the  silica  is  reduced  and  the 
resulting  metalloid  silicon  alloys  with  the  pig-iron.  The  amount  of 
silicon  reduced  depends  on  the  temperature  and  other  conditions  in 
the  furnace.  About  one-half  of  any  manganese  in  the  ore  is  reduced 
and  enters  the  pig-iron,  and  nearly  all  the  phosphorus  in  the  ore 
finds  its  way  into  the  iron. 

The  sulphur  in  the  charge  is  derived  mostly  from  the  coke  which 
contains  about  i  per  cent,  of  that  element.  It  is  highly  desirable 
to  keep  the  sulphur  out  of  the  iron,  and  this  can  be  done  fairly  well 
in  the  blast-furnace  by  the  use  of  an  excess  of  lime  in  the  charge. 
The  lime  is  partly  reduced  to  calcium  which  forms  calcium  sulphide 
and  passes  into  the  slag  thus  removing  the  sulphur  from  the  iron. 
A  high  temperature,  limey  slag  and  excess  of  fuel,  all  assist  the 
removal  of  the  sulphur. 

The  amount  of  fuel  used  in  the  blast-furnace  is  about  equal  to  the 
amount  of  pig-iron  produced.  Coke  is  the  fuel  most  commonly  used, 
but  sometimes  charcoal  is  employed.  Charcoal  contains  far  less 
sulphur  than  coke  does,  and  on  this  account  "charcoal  iron"  can 
be  made  freer  from  sulphur  and  with  less  silicon  than  ordinary 
pig-iron.  The  high  cost  of  charcoal  prevents  its  general  use  in  iron 
smelting.  Anthracite  and  other  non- coking  coals  are  sometimes 
used  in  the  blast-furnace,  but  are  less  suitable  than  coke  or  charcoal. 

THE  ELECTRICAL  PRODUCTION  OF  PIG-IRON 

In  the  blast-furnace  the  fuel  is  used  partly  to  produce  heat  and 
partly  as  a  reagent  for  the  reduction  of  iron  oxide  to  metal.  In 
the  electric  furnace  the  heat  is  furnished  electrically  and  the  car- 
bonaceous fuel  is  needed  merely  for  the  chemical  operation  of 
reducing  oxide  to  metal.  The  amount  of  fuel  needed  for  this 
purpose  is  about  one- third  of  the  amount  needed  for  smelting  the  ore 
in  the  blast-furnace.  The  amount  of  electrical  energy  needed  is 
about  1/4  E.H.P.  year  per  ton  of  pig-iron. 

Comparing  the  cost  of  smelting  by  the  two  methods,  it  may  be 
stated  generally  that  the  blast-furnace  needs  i  ton  of  fuel  for  i  ton 


176  THE  ELECTRIC  FURNACE 

of  pig-iron,  while  the  electric  furnace  needs  1/3  ton  of  fuel  and  1/4 
E.H.P.  year  per  ton  of  pig-iron.  Assuming  that  the  remaining 
items  of  cost  are  about  equal  for  the  two  methods,  it  will  be  seen 
that  for  equal  cost  1/4  E.H.P.  year  should  cost  the  same  as  2/3 
ton  of  fuel,  or  i  E.H.P.  year  should  equal  in  cost  2§  tons  of  fuel. 
Apart  from  other  considerations,  which  will  be  advanced  later,  this 
indicates  that  electrical  smelting  of  iron-ores  to  make  pig-iron  can 
only  be  possible  where  the  electrical  horse-power  year  costs  less 
than  2§  tons  of  fuel. 

THE  ELECTRIC  FURNACE  FOR  IRON  SMELTING 

This  consists  of  a  smelting  chamber  constructed  of  refractory 
materials  and  provided  with  two  or  more  electrodes  usually  made  of 
carbon.  The  electric  current  passes  between  these  electrodes 
through  the  smelting  ore  and  the  resulting  slag  and  metal,  thus 
producing  the  heat  necessary  for  the  operation.  The  charge  of  ore, 
fuel  and  flux  usually  descends  some  kind  of  shaft  or  chute  before 
entering  the  smelting  chamber.  In  the  shaft  it  is  exposed  to  the 
heat  and  reducing  action  of  the  gases  produced  in  the  smelting 
chamber,  and  is  thus  largely  heated  and  reduced  before  reaching  the 
zone  of  fusion. 

Many  types  of  furnaces  have  been  tried  and  there  may  yet  be 
material  changes  in  the  design.  It  seems  advisable,  therefore,  to 
treat  the  subject  historically,  giving  an  account  of  the  various 
experimental  furnaces  that  have  been  constructed  or  designed 
before  the  later  forms  of  furnace  were  evolved. 

The  Heroult  Furnace. — The  experimental  Heroult  furnace,1 
Fig.  78,  as  used  at  Sault  Ste.  Marie  in  the  spring  of  1906,  con- 
sisted of  a  nearly  cylindrical  shaft  in  which  -a  carbon  electrode,  C, 
was  suspended.  The  furnace  was  built  inside  an  iron  casing,  N, 
4  ft.  in  diameter,  bolted  to  a  cast-iron  bottom  plate,  H.  The  lower 
part  of  the  furnace  was  lined  with  carbon,  G,  put  in  as  a  paste,  and  this 
carbon  lining  formed  the  lower  electrode  of  the  furnace,  the  current 
passing  between  C  and  G  through  the  melting  charge.  One  cable 
from  the  transformer  was  connected  to  H,  and  a  number  of  iron 
rods,  7,  served  to  make  better  contact  between  the  bottom  plate  and 
the  carbon  lining.  The  upper  part  of  the  furnace  was  lined  with 
common  fire-bricks,  M,  but  the  carbon  lining  was  continued  to  a  point 

1  Dr.  Haanel,  Report  on  Experiments  at  Sault  Ste.  Marie,  1907,  pp.  3  and  46, 
and  plate  vii,  Paul  L.  T.  Heroult,  U.  S.  patent  858,718;  see  Electrochem.  Industry, 
vol.  v,  p.  325. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE 


177 


a  little  above  the  slag  level,  as  it  resists  the  solvent  action  of  the 
slag  much  better  than  fire-brick.  The  interior  of  the  furnace 
tapered  a  little,  upward  and  downward  from  the  point  at  which  the 
brick  and  carbon  linings  met.  Two  tapping  holes  were  provided, 
the  lower  one  which  leads  to  the  spout,  5,  for  the  pig-iron,  and  the 
upper  one,  D,  for  the  slag.  The  upper  electrode  was  supported  by 
the  holder,  AB,  which  has  already  been  described  (Chapter  IV), 
and  which  was  suspended  by  a  chain  so  that  it  could  be  raised  or 


FIG.  78. — Heroult  ore-smelting  furnace. 

lowered ;  the  regulation  of  the  electrode  would  normally  be  automatic. 
The  electric  current  was  led  to  the  electrode  through  the  holder  AB. 
An  iron  casing  is  very  convenient  in  the  construction  of  any 
kind  of  furnace,  but  for  an  electric  furnace  using  an  alternating 
current,  the  complete  iron  ring,  N,  through  which  the  current  has 
to  pass,  would  be  very  objectionable,  as  it  would  increase  the  in- 
ductance of  the  circuit;  thus  opposing  the  passage  of  the  current, 
and  lowering  the  power  factor.  On  this  account,  a  vertical  strip 
of  the  iron  case,  10  inches  wide,  was  replaced  by  a  copper  plate. 
12 


178  THE  ELECTRIC  FURNACE 

In  operating  the  furnace,  the  current  is  started  between  the 
electrode,  C,  and  the  bottom  of  the  furnace  (a  little  coke  could  be 
placed  in  the  furnace,  if  necessary,  to  prevent  too  large  a  rush  of 
current  on  making  contact),  and  the  ore,  mixed  with  charcoal  and 
fluxes,  is  fed  in  around  the  electrode.  The  heat  generated  by  the 
electric  current  will  heat  the  charge  around  the  end  of  the  electrode, 
and  as  the  charge  becomes  partly  reduced  and  melted  it  will  carry 
the  current  more  readily,  and  the  electrode  can  be  gradually  raised 
until  it  reaches  its  normal  position.  The  part  of  the  furnace 
between  C  and  G  may  be  considered  the  zone  of  fusion,  and  contains 
molten  pig-iron,  F,  molten  slag,  E}  and  a  mixture,  D,  of  charcoal 
and  melting  slag  and  metal. 

The  ore  charged  into  the  furnace  contains  iron  in  an  oxidized 
condition,  and  this  oxide  of  iron  is  reduced  by  the  charcoal,  forming 
metallic  iron  and  carbon  monoxide.  This  direct  reduction  by 
charcoal  probably  takes  place  mainly  in  the  lower  and  hotter  part 
of  the  furnace,  but  the  carbon  monoxide,  so  formed,  is  itself  a  good 
reducing  reagent  and  reacts  with  the  oxides  in  the  upper  part  of  the 
furnace,  partly  reducing  these  and  liberating  carbon  dioxide,  which 
is  again  reduced,  in  part,  to  carbon  monoxide  by  the  charcoal  in 
the  charge.  The  reactions  may  be  represented  as  follows  : 


FeOfC  =  Fe+CO. 


It  will  be  seen  that  the  gas  escaping  from  the  furnace  must  be 
rich  in  carbon  monoxide,  and  is  therefore  more  valuable  than  the 
gas  from  an  ordinary  blast-furnace  which  is  largely  diluted  with 
nitrogen  from  the  blast.  In  the  illustration  this  gas  is  represented 
as  burning  around  the  electrode,  above  the  charge,  but  in  regular 
practice  it  would  be  employed  to  preheat  the  charge.  The  carbon 
monoxide  will  not  reduce  the  iron-ore  until  the  latter  has  become 
somewhat  heated,  and  in  electric  smelting  the  heat  will  not  penetrate 
so  far  up  the  descending  column  of  ore  as  it  does  in  the  blast-furnace, 
as  there  is  a  much  smaller  flow  of  gas  to  carry  the  heat.  The  shafts 
of  electric  smelting  furnaces  will  therefore  not  need  to  be  so  high, 
in  proportion,  as  the  shafts  of  blast-furnaces.  In  Figs.  78  and  79 
the  arrangement  of  the  electrodes  would  also  prevent  a  high  furnace 
from  being  used,  but  this  has  been  modified  in  later  forms  of  the 
furnace,  and  the  volume  of  the  upper  part  of  the  furnace  may  be 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  179 

effectively  increased  if  the  ore  charge  is  preheated  by  the  com- 
bustion of  the  carbon  monoxide. 

Turning  now  to  the  results  obtained  in  this  furnace,  Dr.  Haanel 
reports1  that,  in  the  experimental  runs,  which  were  begun  about 
the  middle  of  January,  1906,  and  continued  until  the  5th  of  March, 
some  55  tons  of  pig-iron  were  electrically  smelted  from  hematite, 
magnetite,  roasted  pyrrhotite,  and  titaniferous  ores.  The  furnace 
worked  satisfactorily  with  all  these  ores,  and  pig-iron,  low  in  sulphur, 
was  obtained  from  the  roasted  pyrrhotite,  and  other  ores  of  high 
sulphur  content.  Charcoal  forms  a  perfectly  satisfactory  reducing 
agent,  and  this  is  important,  since  in  Ontario  and  Quebec  charcoal 
can  often  be  produced  cheaply  from  mill  refuse,  wood  or  even  peat, 
while  coke,  suitable  for  blast-furnaces,  must  be  imported.  In  this 
connection,  it  should  be  remembered  that  the  coke  or  charcoal 
used  in  a  blast-furnace  should  be  of  good  quality,  and  able  to  stand 
the  weight  of  the  heavy  column  of  ore  without  crushing;  while  in 
the  electric  furnace  the  quality  of  the  reducing  reagent  is  less  im- 
portant, and  broken  charcoal  and  partly  charred  wood  was  found 
to  serve  the  purpose.  The  electric  furnace  differs  from  "the  blast- 
furnace in  the  absence  of  a  blast  of  air,  and  in  the  possibility  of 
attaining  a  higher  temperature.  Both  of  these  differences  are  in 
favor  of  the  electric  furnace,  and  cause  it  to  be  a  more  powerful 
reducing  and  melting  appliance  than  the  blast-furnace.  The  strong 
reduction  helps  to  drive  the  sulphur  into  the  slag,  .as  calcium  sul- 
phide, and  the  high  temperature  that  is  attainable  allows  a  very 
limey  slag  to  be  used  for  the  removal  of  the  sulphur.  Strong 
reducing  conditions,  although  desirable  as  removing  the  sulphur, 
have  the  effect  of  increasing  the  amount  of  silicon  in  the  pig-iron, 
and  iron  containing  as  much  as  5  per  cent,  or  6  per  cent,  of  silicon 
was  obtained,  with  only  0.06  per  cent,  of  sulphur  when  smelting  the 
roasted  pyrrhotite.2  Dr.  Haanel  reports,  however,  that  by  increas- 
ing the  limestone  in  the  charge,  the  silicon  in  ferro-nickel  pig  has 
been  lowered  to  2  per  cent.  With  less  sulphurous  ores  the  iron 
could  be  obtained  high  or  low  in  silicon  as  desired,  as  the  degree 
of  reduction  in  the  furnace  is  quite  under  control. 

The  consumption  of  electrical  energy,  in  horse-power  years  per 
long  ton  of  pig-iron,  varied  from  0.268  to  0.333  m  the  later  runs 
on  iron-ores.3  If  the  carbon  monoxide  escaping  from  the  furnace 

1  Preliminary  Report,  1906,  p.  8. 

2  1907  Report,  p.  84. 

3 1907  Report,  runs  12  to  17  in  which  charcoal  was  used. 


180  THE  ELECTRIC  FURNACE 

were  utilized  for  preheating  the  ore  and  flux,  these  figures  would 
be  reduced,  and  somewhat  better  results  may  be  expected 
from  furnaces  of  larger  dimensions,  and  when  the  conditions  for 
smelting  have  been  more  completely  ascertained.  The  amount 
of  charcoal  used  varied  from  30  per  cent,  to  34  per  cent,  of  the 
weight  of  the  ore,  or  about  1,100  to  1,200  Ib.  of  very  poor  charcoal 
per  ton  of  pig. 

After  the  conclusion  of  Dr.  Haanel's  experiments  at  Sault  Ste. 
Marie,  the  plant  was  purchased  by  the  Lake  Superior  Power  Com- 
pany, and  has  been  used  for  the  production  of  ferro-nickel  pig  from 
roasted  pyrrhotite.1 

The  Keller  Furnace  (Fig.  79), 2  differs  from  the  Heroult  in 
having  two  vertical  shafts,  NN',  communicating  below  by  a  pas- 
sage, CC'.  Each  shaft  contains  a  carbon  electrode,  D,  and  the 
current  from  these  electrodes  flows,  normally,  through  the  molten 
metal  K  in  CC ';  but  permanent  carbon  electrodes,  BBf,  connected 
electrically  by  a  copper  bar,  EE',  serve  to  carry  the  current  from 
one  shaft  to  the  other  whenever  the  furnace  is  empty.  If  is  an 
auxiliary  electrode  which  may  be  employed  for  heating  the  metal 
in  K  if  it  should  ever  become  chilled. 

This  furnace  has  the  advantage  of  providing  a  receptacle,  K, 
for  the  molten  metal  and  slag;  the  metal  being  tapped  through 
the  hole,  K,  and  the  slag  through  the  hole,  /.  The  receptacle, 
K,  corresponding  to  the  fore-hearth  or  settler  of  a  copper  furnace, 
receives  the  molten  products  of  two,  or  even  four  shafts,  thus 
reducing  the  labor  of  tapping;  and  the  use  of  two  shafts,  connected 
electrically  in  series,  enables  the  current  to  be  employed  at  a  higher 
voltage  than  in  the  case  of  a  single  shaft-furnace.  The  working 
lining  of  the  furnace  is  made  by  ramming  a  mixture  of  burnt  dolomite 
and  tar  around  a  mold,  and  has  been  found  to  stand  very  well. 
As  the  heat  is  produced  in  the  center  of  the  shaft,  it  should  be 
possible,  by  suitably  proportioning  the  furnace  to  keep  the  walls 
at  so  moderate  a  temperature  that  they  might  be  built  of  ordinary 
fire-brick,  as  in  the  blast-furnace.  Fire-bricks  are,  however,  rapidly 
corroded,  even  at  moderate  temperatures,  by  slags  containing  oxide 
of  iron,  and  would  only  stand  if  the  conditions  were  so  strongly 
reducing  as  to  convert  the  whole  of  this  oxide  to  metal.  It  will  be 
remembered  that  the  working  lining  of  the  Heroult  furnace  was 
carbon,  which  is  infusible  and  does  not  corrode  unless  exposed  to 

1 1907  Report,  pp.  93-95- 

2  Dr.  Haanel's  European  Report,  1904. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE 


181 


oxygen  or  metallic  oxides,  such  as  iron  oxide.  Such  a  lining  will 
last  if  the  furnace  conditions  are  strongly  reducing,  and  pig-iron  is 
being  made,  but  would  not  last  if  it  were  attempted  to  produce 
steel  in  the  furnace,  as  there  would  be  a  considerable  amount  of 
iron  oxide  in  the  slag.  A  basic  lining,  such  as  dolomite,  would  then 
have  to  be  used. 

The  ore  enters  the  furnace  through  iron  hoppers,  MM',  which 
are  provided  with  an  annular  space,  Ly  into  which  the  gases  from 


FIG.  79. — Keller  furnace. 

N  can  easily  escape  instead  of  passing  up  through  the  ore  in  M. 
From  L  the  gases  are  withdrawn  in  pipes  and  utilized  in  any  suitable 
manner,  such  as  driving  a  gas  engine  or  preheating  the  ore.  The 
iron  casing,  round  the  furnace  inspected  by  Dr.  Haanel,  was  the 
cause  of  a  very  low  power-factor  being  obtained,  and  it  will  be 
omitted  or  modified  in  the  future. 

The  Haanel  Commission  visited  the  works  of  Messrs.  Keller, 
Leleux  &  Co.,  at  Li  vet,  France,  in  March,  1904,  and  during  their 
visit  some  30  tons  of  ore  were  smelted  electrically.1  The  ore  was 

1  European  Report,  pp.  90-109. 


182  THE  ELECTRIC  FURNACE 

hematite  and  contained  48.7  per  cent,  of  iron  and  10  per  cent, 
of  moisture.  Coke,  containing  7.6  per  cent,  of  ash  and  91.1  per  cent, 
of  fixed  carbon,  was  used  for  reducing  the  ore,  and  the  amount 
required  varied  from  about  18  per  cent,  to  20  per  cent,  of  the  ore, 
from  17  per  cent,  to  19  per  cent,  of  the  ore  and  fluxes,  or  from  800 
to  900  Ib.  per  ton  of  pig-iron.  The  energy  used,  per  ton  of  pig, 
was  0.532  E.H.P.  years  in  the  first  experiment,  and  o.  253  E.  H.  P. 
years  in  the  second  experiment.  In  the  first  experiment  the  furnace 
was  working  badly,  and  the  experiments  at  Sault  Ste.  Marie  tend 
to  show  that  the  smaller  of  these  figures  may  be  considered  reliable. 
The  Harmet  Furnace  (Fig.  So),1  differs  from  the  Heroult  and 
Keller  furnaces  in  having  the  electrodes  inserted  laterally  into  the 
lower  part  of  the  shaft  instead  of  passing  vertically  down  the  furnace. 
The  shaft,  61,  is  enlarged  below  to  allow  of  the  insertion  of  the 
electrodes,  EE,  and  the  current  passes  between  these  through  the 
melting  charge,  the  slag,  C,  and  the  molten  metal,  B.  The  inclined 
lateral  electrodes  will  probably  be  less  satisfactory  in  actual  use 
than  a  central  electrode,  because  it  will  not  be  easy  to  regulate 
the  current  by  raising  or  lowering  them  as  is  done  in  the  other 
furnaces;  supporting  the  electrodes  in  this  position  will  also  be  less 
easy,  and  the  walls  will  be  apt  to  melt  around  the  electrodes.  On 
the  other  hand  the  height  of  the  shaft,  S,  is  not  limited  as  in  the 
Heroult  furnace,  by  the  length  of  the  electrode;  and  better  provision 
can  be  made  for  the  preheating  and  reduction  of  the  ore.  Harmet 
utilizes  the  combustible  gases  escaping  from  the  top  of  the  shaft, 
for  burning,  in  a  separate  furnace  or  calciner,  in  which  the  ore  is 
calcined  and  preheated  before  being  charged  into  the  main  furnace. 
Some  of  the  gas  is  returned  to  the  foot  of  the  shaft,  being  blown  in 
at  this  point  to  supply  a  reducing  gas  for  converting  the  iron  oxide 
to  metal,  and  to  carry  some  of  the  heat  from  the  crucible  up  the 
shaft,  so  as  to  preheat  and  reduce  the  descending  ore.  The  use 
of  the  gas  to  preheat  the  ore  before  charging  into  the  furnace  is 
very  desirable,  but  there  will  be  no  need  to  blow  gases  through  the 
smelting  shaft,  because  reducing  gases  are  always  formed  here  in 
large  amount,  and  because  the  combustion  of  the  gas  in  the  calciner 
would  heat  the  ore  to  a  temperature  at  which  it  would  begin  to  be 
reduced  to  the  metallic  state  directly  it  was  introduced  into  the 
smelting  shaft. 

1  Treatise  on  Electro-metallurgy  of  Iron,  by  Henri  Harmet,  European  Report, 
1904,  pp.  124-164.     Electrochemical  Industry,  vol.  i  (1903),  p.  422. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE 


183 


Mr.  Henri  Harmet  has  written  a  treatise  on  the  electro-metallurgy 
of  iron,  which  is  printed  in  Dr.  Haanel's  European  Report,  and  in 
this  he  considers  every  conceivable  way  in  which  iron  ores  can  be 
reduced  by  the  joint  use  of  carbon  and  electrical  heat,  but  no  mention 
is  made  of  any  actual  furnace  embodying  his  views — even  on  the 
experimental  scale. 


////7//^///////7/^ 

FIG.  80. — Harmet  furnace. 

The  Haanel-Heroult  Furnace,  shown  in  Fig.  Si,1  is  an  improve- 
ment on  Heroult's  earlier  furnace.  The  upper  electrode  no  longer 
descends  through  the  same  shaft  as  the  ore,  but  a  separate  opening 
is  provided  for  it  into  the  smelting  zone  of  the  furnace;  while  two 
lateral  shafts  are  provided  for  the  heating  and  reduction  of  the  ore. 
The  ore  shafts,  A  and  B,  can  thus  be  made  of  any  desirable  height, 
not  being  limited  by  the  length  of  the  electrode;  and  hoppers,  K  K, 
can  be  used  for  charging  the  ore,  thus  allowing  the  combustible 
gases  to  be  led  away  through  pipes,  L  L,  for  preheating  the  ore  or 

1  Dr.  Haanel's  Sault  Ste.  Marie  Report,  1907,  plate  ix,  and  pp.  92-93. 


184 


THE  ELECTRIC  FURNACE 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  185 

other  purposes.  The  electrode,  C  D,  also,  is  protected  from  heat 
and  wear  except  at  the  working  end,  C. 

The  stuffing-box,  F,  through  which  the  electrode  enters  the 
furnace,  is  needed  to  prevent  the  escape  of  gases.  It  is  made  of 
copper,  is  water-cooled,  and  is  packed  with  wedge-shaped  rings  of 
graphite.  The  graphite  packing  not  only  makes  a  gas-tight  joint, 
but  also  ensures  an  electrical  contact  between  the  electrode  and  the 
stuffing-box,  so  that  the  electric  current  can  be  led  to  the  electrode 
by  the  arm,  G.  It  should  be  noted  that  this  use  of  the  stuffing-box 
for  electrode  holder  not  only  makes  it  serve  a  double  purpose,  but, 
by  leading  the  current  into  the  electrode  as  near  as  possible  to  its 
working  end,  does  away  with  all  needless  production  of  heat  by  the 
passage  of  the  current  through  the  electrode.  The  furnace  is  cased 
with  steel  plates,  but  the  top,  O,  and  a  strip  at  one  side,  P,  as  well  as 
the  stuffing-box,  are  made  of  copper,  so  as  to  avoid  a  complete 
ring  of  iron  around  the  path  of  the  current. 

The  furnace  is  shown  filled  with  ore,  flux  and  charcoal,  as  it  would 
be  during  operation,  and  with  molten  slag  and  metal  at  S  and  M. 
These  are  drawn  off  through  three  tapping  holes  and  spouts,  of 
which  the  middle  and  lowest  spout  is  for  metal,  while  the  other  two 
are  for  slag.  The  shafts  and  other  parts  of  the  furnace  are  lined  with 
fire-bricks,  but  the  part,  N,  which  is  exposed  to  the  action  of  melting 
ore,  slag  and  metal,  is  composed  of  specially  refractory  material, 
such  as  magnesite.  The  arch  across  the  middle  of  the  furnace  will 
also  be  particularly  liable  to  corrosion  and  wear,  but  will  be  some- 
what protected  by  the  cooling  effect  of  the  stuffing-box. 

The  lower  electrode,  £,  consists,  as  in  the  earlier  furnace,  of  a 
rammed  carbon  plug,  making  contact  with  the  aid  of  iron  spikes 
to  the  heavy  cast-iron  bottom  plate  and  so  to  the  contact  piece,  /. 
The  upper  electrode  is  made  cylindrical,  to  allow  of  its  passage 
through  the  stuffing-box.  Additional  lengths,  Z>,  are  attached  by 
threaded  joints  as  shown  in  section  in  the  figure,  thus  avoiding  any 
interruption  in  operation  or  waste  of  electrode.  The  piece  R, 
clamped  on  the  electrode,  serves  to  hold  it  while  a  new  piece  is  being 
screwed  on,  and  also  for  raising  or  lowering  the  electrode. 

No  scale  is  given  in  the  original  drawing,  which  is  merely  intended 
to  show  the  principles  on  which  the  furnace  would  be  constructed. 

The  Turnbull-Heroult  Furnace,  Fig.  82, 1  is  a  modification  of 
Heroult's  original  furnace  which  has  been  devised  by  his  Canadian 
representative,  Mr.  R.  Turnbull.  As  shown  in  the  figure  there  are 

1  Dr.  Haanel's  Sault  Ste.  Marie  Report,  1907,  plate  xviii,  and  p.  147. 


186 


THE  ELECTRIC  FURNACE 


FIG.  82.— Turnbull-Heroult  furnace. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE 


187 


six  movable  electrodes,  descending  into  a  smelting  groove  or  canal, 
which  forms  a  closed  rectangle.  The  ore  descends  in  a  central 
shaft,  and  is  distributed  to  the  smelting  groove  by  six  inclined 
shoots,  one  descending  between  each  adjacent  pair  of  electrodes. 
The  number  of  electrodes  is  preferably  some  multiple  of  three,  so  as 
to  permit  the  use  of  three-phase  current. 

The  three  electrical  connections,  a,  b,  and  c,  on  the  bottom  of 
the  furnace,  appear  to  indicate  that  the  secondary  windings  of  the 
three  transformers  are  not  connected  together,  but  that  the  cables 
from  one  end  of  each  are  connected  to  A  A,  B  B,  and  C  C,  respec- 
tively, while  the  return  cables  are  all  connected  to  the  common 
terminal  a  b  c,  on  the  bottom  plate  of  the  furnace.  The  wiring  for 
this  arrangement  is  shown  in  Fig.  83,  in  which  Xd,  Yd,  and  Zd  are 


FIG.  83. — Connections  for  Turn- 
bull-Heroult  furnace. 


FIG.  84. — Connections  for  Turn- 
bull-Heroult  furnace. 


the  secondary  windings  of  the  transformers,  each  of  which  is  con- 
nected to  the  furnace  by  two  cables,  one  leading  to  a  pair  of  movable 
electrodes  and  the  other  to  the  bottom  of  the  furnace.  It  will  be 
evident  that  by  connecting  the  secondary  windings  in  Y  form  as  in 
Fig.  84,  the  return  cables  from  a  b  c  to  D  will  be  unnecessary,  as 
each  cable  and  pair  of  electrodes  will  serve  as  a  return  for  the  other 
cables  and  electrodes.  Thus  the  current  entering  the  furnace  by 
the  electrodes  A  A  will  pass  down  to  the  bottom  of  the  furnace 
and  pass  up  again  by  the  electrodes  BB  and  CC.  This  arrange- 
ment will  save  both  the  cost  of  the  return  cables,  and  the  electrical 
energy  wasted  in  them.  It  might,  however,  be  desirable  to  use  a 
single  return  cable  between  a  b  c  and  D  to  provide  for  any  unbal- 
anced current,  as  in  the  operation  of  replacing  one  of  the  electrodes. 


188  THE  ELECTRIC  FURNACE 

When  the  furnace  is  once  in  regular  operation  the  current  will  be 
carried  from  one  electrode  to  another  through  the  molten  iron  in  the 
smelting  channel,  without  needing  to  pass  into  the  carbon  bottom  of 
this  channel,  and  the  carbon  bottom  might  therefore  be  omitted.  In 
a  furnace  using  three-phase  current,  as  represented  in  Figs.  83  or  84, 
a  small  proportion  of  the  current  will  pass  from  A  to  B,  or  from  B 
to  C,  directly  through  the  charge  without  passing  through  the 
molten  metal  or  the  bottom  of  the  furnace.  In  these  furnaces  the 
voltage  between  A  and  B,  or  between  B  and  C,  will  be  1.73  times  the 
voltage  between  A  and  a,  or  between  B  and  b;  and  if  the  movable 
electrodes  were  near  together,  surrounded  by  a  deep  layer  of  charge, 
and  raised  considerably  above  the  bottom  of  the  furnace,  the  bulk 
of  the  current  might  pass  directly  between  them,  and  the  metal 
in  the  bottom  of  the  furnace  might  become  too  cold  or  even  solidify. 
In  the  Turnbull  furnace,  Fig.  82,  there  would  be  no  danger  of  this 
as  the  electrodes  are  widely  separated  from  each  other,  and  are  not 
raised  very  high  above  the  metal  in  the  furnace. 

The  upper  part  of  the  furnace,  Fig.  82,  is  designed  to  utilize  the 
combustible  furnace  gases  for  preheating  the  ore  and  limestone. 
This  cannot  be  done  in  the  main  shaft  of  the  furnace,  for  if  air  were 
introduced  there  to  burn  the  gas,  it  would  also  burn  the  charcoal 
or  other  fuel  in  the  ore  mixture.  A  lateral  rotating  tube,  T  T,  is 
therefore  provided,  down  which  the  ore  gradually  passes.  The 
combustible  gases  from  the  furnace  burn  in  this  tube,  air  being  in- 
troduced through  the  bent  pipe  P;  and  the  products  of  combustion 
escape  by  the  flue  F.  The  charcoal  or  other  fuel  is  introduced 
through  the  hopper  H,  and  is  thus  protected  from  the  burning  gas 
and  air. 

The  preheating  of  the  ore  and  limestone  in  the  tube  T  T  has 
several  advantages.  It  calcines  the  limestone,  removing  the  car- 
bon dioxide  which  would  otherwise  rob  carbon  from  the  fuel;  it 
roasts  the  ore,  removing  a  part  of  any  sulphur  it  may  contain  and 
leaving  it  in  a  better  condition  for  the  smelting  operation;  and  the 
ore,  by  being  heated,  is  fitted  for  immediate  reduction  to  the  me- 
tallic state  when  it  enters  the  reducing  atmosphere  of  the  furnace, 
as  well  as  gaining  an  amount  of  heat  which  would  otherwise  have 
to  be  furnished  by  the  electric  current.  This  preheating  of  the  ore 
is  not  of  great  importance  in  a  blast-furnace,  where  an  ample  supply 
of  heat  is  carried  up  by  the  blast  and  serves  to  preheat  an  immense 
volume  of  ore  to  an  increasingly  high  temperature  as  it  descends 
in  the  furnace;  but  in  the  electric  furnace  only  a  small  amount  of 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  189 

heated  gas  rises  from  the  smelting  zone  to  heat  the  descending  ore, 
and  the  preheating  of  the  ore  is  therefore  very  desirable. 

In  the  figure,  the  electrodes  are  shown  hanging  freely  in  the 
furnace,  but  it  is  intended  to  have  some  form  of  stuffing-box  to 
prevent  the  escape  of  gas.  Each  electrode  would  also  need  to  be 
insulated  from  the  metal  casing  of  the  furnace.  The  necessary 
supports  and  gearing  for  the  tube,  T  Ty  are  omitted  in  the  drawing. 
The  metal  and  slag  are  drawn  off  through  suitable  spouts  which 
are  shown. 

A  2,ooo-h.p.  furnace1  of  similar  type  was  erected  at  Heroult, 
California,  for  smelting  a  rich  magnetite  ore  with  charcoal.  The 
furnace  had  a  guaranteed  output  of  20  tons  a  day.  The  location 
is  favorable  for  electric  smelting  on  account  of  the  abundant  water- 
power  and  the  high  price  of  pig-iron,  and  of  fuel  suitable  for  use  in 
the  blast-furnace.  The  furnace  was  formally  started  on  the  4th 
of  July,  1907,  before  the  electrical  equipment  was  thoroughly  com- 
pleted. It  made  some  iron,  7  tons  being  drawn  on  the  i7th 
July,2  but  for  steady  work  more  electric  power  was  required.  In- 
formation with  regard  to  the  furnace  was  given  in  the  Mining  and 
Scientific  Press  of  July  20,  1907,  and  in  the  Electrochemical  Industry, 
vol.  v,  p.  318,  from  which  the  following  particulars  are  taken:  The 
ore  was  a  magnetite  containing  about  70.2  per  cent.  Fe.,  0.012  per 
cent.  S.,  o.oi  per  cent.  P.,  2.4  per  cent.  SiOz  and  insoluble.  Good 
limestone  for  flux  was  also  available.  The  ore  was  expected  to 
cost  $1.50  per  ton  delivered  to  the  smelter,  and  the  electric  power, 
$12  per  horse-power-year.  The  best  pig-iron  was  selling  at  $30 
or  $32  per  ton  in  San  Francisco,  and  it  was  expected  that  the  elec- 
tric pig-iron  could  be  made  and  delivered  there  at  a  cost  of  from 
$15  to  $18  per  ton.  In  this  furnace  there  were  three  electrodes 
supplied  with  three-phase,  6o-cycle  current  at  50  volts;  the  amount 
of  current  used  being  stated  as  30,000  amperes.3 

Possibilities  in  Electric  Smelting. — The  early  experiments  on 
the  electric  smelting  of  iron-ores  at  Sault  Ste.  Marie  and 
elsewhere,  were  all  hampered  by  inadequate  electrical  equip- 
ment, by  the  small  scale  of  the  furnace,  and  by  the  fact 
that  no  use  was  made  of  the  escaping  furnace-gases.  It  is  very 
desirable  to  know  what  improvement  in  efficiency  may  be  expected 
when  all  possible  improvements  have  been  made  in  the  design  and 

1  Dr.  Haanel,  Report,  1907,  p.  148. 

2  Engineering  and  Mining  Journal,  August  10,  1907,  p.  278. 

3  For  further  particulars  of  this  furnace  see  p.  203. 


190  TEE  ELECTRIC  FURNACE 

construction  of  the  electric  smelting  furnace,  and  what  is  the  mini- 
mum amount  of  fuel  and  electrical  energy  that  will  then  be  needed. 
For  this  purpose  the  operation  of  an  ideal  furnace  may  be  studied, 
omitting  for  the  present  any  consideration  of  how  such  a  furnace 
could  actually  be  constructed. 

The  ideal  furnace  shown  in  Fig.  85  consists  of  a  smelting  shaft 
divided  by  imaginary  planes,  aa,  and  bb,  into  three  distinct  zones, 
Ay  By  and  C.  The  ore  and  limestone  are  introduced  at  the  top  of 
the  shaft  and  are  roasted  and  preheated  by  the  gases  leaving  the 
zone  By  which  are  burned  in  C  by  air  introduced  at  bb.  In  the  zone 
By  the  roasted  and  preheated  ore  is  partly  reduced  by  the  reducing 
gases  leaving  the  zone  Ay  enough  combustible  gas  being  left  to 
preheat  the  ore  in  C.  In  the  lowest  zone,  C,  carbonaceous  fuel 
introduced  at  aa,  serves  to  complete  the  reduction  of  the  ore  to  the 
metallic  state,  generating  at  the  same  time  reducing  gases  which 
pass  up  the  furnace,  and  to  carburize  the  resulting  iron;  while  the 
necessary  heat  is  produced  by  electrical  energy  introduced  for  ex- 
ample by  the  electrodes,  E  E.  The  figure  merely  serves  to  show  the 
principles  of  an  ideal  furnace  as  clearly  as  possible;  any  actual  furnace 
embodying  these  principles  would  be  constructed  quite  differently. 

It  is  well  known  that  in  the  iron  blast-furnace  the  efficiency  is 
limited  by  the  composition  of  the  escaping  gases,  at  least  half 
the  carbon  that  is  burnt  in  the  furnace  escaping  in  the  half-con- 
sumed form  of  carbon  monoxide.  The  same  is  true  of  any  simple 
electric  sm  el  ting-furnace,  such  as  Heroult's  experimental  furnace 
in  which  the  charcoal  was  introduced  with  the  ore  at  the  top 
of  the  furnace.  If  now  the  carbon  monoxide  escaping  from  such  a 
furnace  is  burnt  and  used  to  preheat  the  ore,  a  certain  saving  of 
electrical  energy  would  be  obtained,  but  there  would  be  no  sav- 
ing of  fuel,  and  the  burning  of  the  waste  gases  would  sometimes 
furnish  more  heat  than  was  needed  for  preheating  the  ore,  thus 
leading  to  waste  and  overheating  of  the  top  of  the  furnace.  In 
the  ideal  furnace  of  Fig  85,  part  of  the  waste  gases  are  used  for  a 
partial  reduction  of  the  ore  in  zone  B,  and  the  remainder  is  em- 
ployed for  preheating  in  zone  C.  In  this  way  the  greatest  possible 
economy  in  both  fuel  and  electrical  energy  can  be  obtained.  As 
the  fuel  is  used  in  this  furnace  both  for  reduction  and  for  heating, 
it  will  be  possible,  within  certain  limits,  to  use  rather  more  fuel  and 
less  electrical  energy,  or  less  fuel  and  more  electrical  energy,  obtain- 
ing in  both  cases  perfect  combustion  and  economy,  and  the  relative 
price  of  the  two  commodities  would  decide  which  to  employ. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  191 


Ore  and 
Limestone 


Escaping 
Gases 


3,  CaCOs 
Si02,  AI203 


N,  COZ. 
HzO.etc. 


Ore  is  dried  and  heated. 
Limestone  is  calcined. 


Heat    produced 
by  burning   CO, 


Heafed   ore  is 
partly  reduced  by  CO. 


FeO,  CaO 


"°IL!'_  f 

r~  i 

T  CO 


Reduction  of  ore 
to  metal  by  C. 


Electrical 
Heating 


Molten  Slag  (SiOZl  CaO,  A1205) 


Molten  Pig(Fe,  C,  5i  etc.) 


CaGOs-CaO*COt 


FeO^C-  Fe*CO 


FIG.  85.— Ideal  furnace  for  smelting  iron  ores. 


192  THE  ELECTRIC  FURNACE 

A  simple  example  will  demonstrate  the  action  of  the  furnace. 
Suppose  that  pure  hematite  ore,  Fe203,  is  charged  in  at  the  top 
of  the  furnace  and  that  pure  carbon  equal  to  15  per  cent,  of  the 
weight  of  the  ore  (two  atoms  of  carbon  to  each  molecule  of  ferric 
oxide),  is  charged  at  aa,  together  with  as  much  additional  carbon  as 
is  needed  to  carburize  the  iron.  The  ore,  preheated  in  C,  will  be 
reduced  to  FeO  in  B,  and  in  A,  the  FeO  will  be  reduced  to  metallic 
iron.  The  equations  show  how  this  works  out,  and  that  on  enter- 
ing C,  half  of  the  carbon  will  have  been  fully  burnt,  and  half  will 
be  fn  the  form  of  carbon  monoxide. 


In  A,     2FeO+2C  =  2Fe+2CO. 
In  B,     Fe2O3+2CO 
In  C,     CO+0  =  C02. 

The  heat  value  of  the  carbon  monoxide  burning  in  C  is  35  per 
cent,  of  the  original  heat  value  of  the  carbon,  and  this  with  the  heat 
carried  up  by  the  furnace  gases  would  heat  the  ore  to  about  1,500° 
C.,  which  would  be  needlessly  high.  If  on  the  other  hand  the  car- 
bon were  reduced  to  about  n  per  cent,  of  the  ore  (three  atoms 
of  carbon  to  two  molecules  of  ferric  oxide)  the  whole  of  the  car- 
bon would  be  required  for  reduction,  leaving  nothing  for  preheating. 
Deciding  on  some  proportion  of  carbon  between  n  per  cent,  and 
15  per  cent,  of  the  ore,  it  would  be  possible  to  calculate  how  much 
electrical  energy  would  be  needed  to  supply  the  remainder  of  the 
heat  for  smelting. 

In  the  experiments  that  have  been  made  in  electric  smelting,  non- 
volatile fuel  such  as  coke  or  charcoal  has  been  employed,  because 
the  volatile  matter  arising  from  a  fuel  like  soft  coal  would  not  only 
be  wasted  but  would  have  made  the  operation  of  the  furnace  decidedly 
unpleasant.  In  the  ideal  furnace,  ample  provision  is  made  for  the 
use  of  carbonaceous  gases  in  the  zones  B  and  C,  and  any  kind  of  fuel, 
even  oil  or  natural  gas,  could  be  used  effectively  if  introduced  at  the 
point  aa. 

The  fuel  entering  the  ideal  furnace  is  completely  burnt  before 
it  leaves  the  furnace,  and  the  whole  value  of  it  and  of  the  electrical 
energy  may  be  communicated  to  the  charge.  The  fuel  is  used  in 
part  for  the  chemical  work  of  reducing  the  oxides  to  metal  and 
carburizing  the  resulting  iron,  and  the  heat  from  the  remainder  of  the 
fuel  and  from  the  electrical  energy  is  used  in  part  to  furnish  heat  for 
the  chemical  changes  that  must  be  effected  in  the  ore,  and  in  part 
leaves  the  furnace  in  the  molten  metal  and  slag,  in  the  gases  escap- 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  193 

ing  from  the  top  of  the  furnace,  and  by  conduction  through  the 
walls  of  the  furnace.  The  heat  consumed  in  chemical  reactions 
is  an  essential  part  of  the  operation,  the  heat  carried  out  by  the 
molten  slag  and  metal  is  usually  considered  to  be  an  unavoidable 
loss,  though  some  of  this  might  be  recovered  if  it  were  worth  while, 
the  heat  escaping  in  the  gases  at  the  furnace  top  may  be  reduced 
to  a  very  small  proportion  of  the  whole,  and  the  loss  by  conduction 
through  the  walls  can  be  reduced  to  a  moderate  proportion  in  well- 
built  furnaces  of  large  dimensions. 

A  few  examples  will  now  be  given  to  show  what  will  be  the  minimum 
amount  of  fuel  and  electrical  energy  needed  for  smelting  an  iron-ore 
in  such  a  furnace. 

The  first  example  is  one  given  by  Prof.  Richards1  and  shows  how 
much  electrical  energy  and  good  charcoal  would  be  needed  to  smelt 
a  magnetite  ore,  obtaining  a  gray  pig-iron. 

The  magnetite  ore  contains: 

Fe2O3 60.  74  per  cent.  MgO 5. 50  per  cent 

FeO 17.18  per  cent.  P2O5 0.04  per  cent. 

SiO2 6.60  per  cent.  S 0.57  per  cent. 

A12O3 i .  48  per  cent.  CO2 2 . 05  per  cent. 

CaO 2.84  per  cent.  H20 3.00  per  cent. 

It  is  to  be  mixed  with  a  good  variety  of  charcoal,  assumed  90  per 
cent,  carbon  and  10  per  cent,  moisture,  and  with  enough  pure  silica 
sand  to  make  a  slag  with  33  per  cent,  silica.  The  pig-iron  is  to 
contain  4  per  cent,  carbon,  3.5  per  cent,  silicon,  and  92.4  per 
cent,  of  iron.  One  ton  of  pig-iron  will  require  1.654  tons  of  iron- 
ore  for  its  production.  Taking  first  the  case  in  which  the  gases 
entering  zone  C  contain  two  volumes  of  CO 2  to  one  volume  of  CO. 
The  carbon  needed  for  one  metric  ton  of  pig  will  be  224  kg.;  that  is 
249  kg.  or  550  Ib.  of  charcoal.  The  electrical  energy  required  will 
depend  upon  how  much  heat  is  lost  by  conduction  and  radiation 
from  the  furnace,  and  in  the  escaping  gases.  Supposing  first  that 
the  gases  are  quite  cold,  and  that  no  heat  is  lost  by  radiation,  etc. 
the  electrical  energy  needed  would  be  about  0.13  horse-power-years 
per  metric  ton  of  pig,  while  if  the  more  reasonable  assumption  were 
made  that  the  gases  escaped  at  300°  C.,  and  that  the  losses  by  con- 
duction and  radiation  from  the  furnace  were  20  per  cent,  of  the  heat 
generated,  (that  is  of  the  electrical  heat  and  of  the  heat  produced 
by  the  gases  burning  in  the  zone  C),  0.20  horse-power-years  of 
electrical  energy  would  be  needed.  In  this  case  the  heat  produced 

1  Richards  Metallurgical  Calculations,  vol.  ii,  Problem  76,  p.  404. 

13 


194  THE  ELECTRIC  FURNACE 

in  C  by  burning  gases  was  about  30  per  cent,  of  the  heat  produced  elec- 
trically. It  would  be  possible  to  use  rather  less  charcoal  and  more 
electrical  energy,  or  less  electrical  energy  and  more  charcoal 
than  indicated  in  this  example,  but  taking  these  figures  as  fairly 
typical  of  the  amount  of  good  charcoal  and  of  electrical  energy 
actually  employed  in  the  furnace,  it  will  be  necessary  to  make  cer- 
tain additions  if  the  results  are  to  represent  working  conditions. 
Thus  to  the  electrical  energy  must  be  added  the  losses  in  trans- 
formers, cables  and  connections,  say  10  per  cent.,  raising  the  figure 
to  0.22  horse-power-years,  and  a  further  addition  must  be  made  to  al- 
low for  the  fact  that  the  furnace  will  not  be  operated  continuously 
during  the  year,  and  that  even  when  it  is  running  it  will  not  always 
draw  the  full  power  for  which  payment  is  made.  In  this  connection  it 
will  not  be  necessary  to  consider  the  time  when  the  furnace  may  be 
out  of  work  for  long  periods  for  repairs,  as  provision  would  be  made 
by  having  a  spare  furnace,  to  employ  the  power  as  regularly  as  pos- 
sible. A  certain  loss  of  charcoal  will  occur  through  mechanical  losses, 
and  it  will  probably  be  safe  to  allow  600  Ib.  of  charcoal  and  0.25 
E.H.P.  years  per  ton  of  pig-iron  as  the  final  solution  of  the  above 
problem. 

As  another  example  may  be  taken  the  thirteenth  experimental 
run  with  the  Heroult  furnace  at  Sault  Ste.  Marie  in  February, 
1906. 1  The  run  lasted  61  1/2  hours,  the  mean  current  was  5,000 
amperes  at  35.7  volts,  with  a  power  factor  of  0.919,  giving  164 
as  the  mean  kilowatts  during  the  run.  12,858  Ib.  of  pig-iron  were 
obtained  with  a  consumption  of  1,140  Ib.  of  charcoal  and  0.268  E.H.P 
years  per  ton  of  pig. 

The  ore  was  magnetite  from  Wilbur  mine,  containing: 

Si02 6.20  per  cent.     MgO 6.84  per  cent. 

Fe2O3 55-42  per  cent.     MnO o.  26  per  cent. 

FeO 23.04  per  cent.     PzOs 0.023  per  cent. 

(Fe 56.69  per  cent.)    (P o.oi  per  cent.) 

A12O3 2.56  per  cent.     S 0.05  per  cent. 

CaO 2 .  oo  per  cent.     CO2,  etc 3.61  per  cent. 

The  charcoal  contained  14.0  per  cent,  moisture,  27.56  per  cent, 
volatile  matter,  55.9  per  cent,  fixed  carbon,  2,54  per  cent,  ash,  and 
0.058  per  cent,  sulphur. 

1  Dr.  Haanel,  1907  Report,  p.  46.  The  figures  in  the  Report  refer  to  the  2,000- 
Ib.  ton  of  pig-iron.  In  this  book  the  author  has  adopted  the  long  ton  of  pig-iron, 
and  occasionally  the  metric  ton  which  is  almost  identical,  as  agreeing  more  gen- 
erally with  commercial  practice. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  195 

During  the  run  21,150  Ib.  of  ore  was  smelted  with  6,555  lb.  °f 
charcoal,  and  1,191  Ib.  of  sand  for  flux.  The  sand  contained 
81.71  per  cent,  of  silica,  and  14.27  per  cent,  of  alumina,  with  1.6  per 
cent,  of  lime  and  i.n  per  cent,  of  magnesia. 

The  mean  analysis  of  the  pig-iron  was : 

Si,  1.75  per  cent.;  S.  0.029  per  cent.;  P,  0.022  per  cent.;  Mn, 
0.23  per  cent.;  C,  4.58  per  cent. 

Supposing  that  the  ore  were  smelted  in  a  simple  furnace  such  as 
was  actually  used,  in  which  the  ore,  flux  and  charcoal  are  all  charged 
into  the  furnace  at  the  top,  and  no  use  is  made  of  the  escaping  gases, 
it  will  be  necessary  to  make  some  assumption  in  regard  to  the  com- 
position of  these  gases  as  no  information  is  given.  Assuming  that 
they  consisted  of  equal  volumes  of  CO  and  COz,  it  will  be  found  that 
the  carbon  required  to  reduce  the  ore  and  carburize  the  pig-iron  will 
be  14  per  cent,  of  the  ore,  which  will  correspond  to  25  per  cent,  of 
charcoal,  or  930  Ib.  of  charcoal  per  ton  of  pig.  In  the  actual  case 
1,140  Ib.  were  used,  part  of  which  was,  however,  burned  on  the  top 
of  the  charge.  Assuming  further  that  the  gases  escape  at  400°  C., 
and  that  20  per  cent,  of  the  electrical  heat  is  wasted  by  radiation  and 
conduction  from  the  furnace,  a  calculation  showed  that  0.267 
E.H.P.  years  per  ton  of  iron  would  be  needed,  a  figure  which  agrees 
better  than  could  be  expected  with  thex  amount  actually  used, 
which  was  0.268. 

If  now  the  same  charge  were  smelted  in  the  ideal  furnace,  so  that 
the  escaping  gases  were  utilized  to  preheat  the  charge,  and  allowing 
for  the  loss  of  20  per  cent,  of  the  electrical  heat  and  20  per  cent,  of 
the  heat  produced  by  the  burning  gases,  it  will  be  found  that  only 
0.216  E.H.P.  years  would  be  needed. 

In  this  calculation  only  the  fixed  carbon  in  the  charcoal  has 
been  considered,  but  with  the  ideal  furnace  the  volatile  matter  in 
the  charcoal  would  also  be  of  use  for  reducing  and  preheating 
the  ore  in  the  upper  zones  of  the  furnace.  A  smaller  amount  of 
charcoal  and  electrical  energy  would  therefore  be  sufficient. 

In  conclusion  it  may  be  stated  that  in  an  electric  furnace  of 
good  construction,  one  ton  of  pig-iron  should  be  obtained  with 
the  use  of  600  to  800  Ib.  of  charcoal  and  about  0.20  to  0.22  E.H.P. 
years,  and  that  in  order  to  allow  for  delays  the  amount  of  electrical 
energy  should  be  raised  to  about  0.25  E.H.P.  years.1 

1  These  figures  apply  to  ores  of  50  per  cent,  or  60  per  cent,  of  iron.  For  poorer 
ores  a  larger  amount  of  electrical  energy  would  be  needed,  but  the  amount  of 
charcoal  per  ton  of  pig  would  not  be  much  increased. 


196  THE  ELECTRIC  FURNACE 

The  only  furnace  illustrated  in  these  pages  in  which  the  escaping 
gases  are  used  to  preheat  the  charge  is  the  Turnbull  furnace,  Fig.  82. 
The  main  part  of  this  furnace  corresponds  to  zone  A  of  the  ideal 
furnace  and  the  preheating  tube  to  zone  C.  There  is  thus  nothing 
corresponding  to  zone  B,  in  which  the  gases  from  the  lower  part  of 
the  furnace  can  exercise  their  reducing  action  on  the  preheated 
ore.  It  remains  to  be  seen  whether  this  zone  will  be  required  in 
practice. 

After  devising  the  ideal  furnace  of  Fig.  85,  in  which  the  greatest 
advantage  is  taken  of  the  fuel  and  of  the  electrial  power,  the  author 
found  that  it  had  already  been  invented  and  patented  by  Paul 
Heroult,1  who  introduces  the  air  by  tuyeres  at  bb,  and  supplies  the 
fuel  by  a  vertical  tube  down  the  center  of  the  furnace  to  the  level  aa. 

Collecting  the  results  that  have  been  obtained  in  the  electrical 
production  of  pig-iron  from  the  ore,  it  may  be  stated  that  the 
process  is  technically  successful,  and  gives  better  results  .than 
the  blast-furnace  in  regard  to  the  use  of  sulphurous  ores,  titanif- 
erous  and  similar  refractory  ores,  and  ores  in  a  state  of  powder 
such  as  iron  sand,  or  ores  which  have  been  concentrated  by  magnetic 
or  similar  processes.  The  process  also  allows  of  the  use  of  inferior 
and,  therefore,  cheaper  fuel.  The  power  required  is  about  1/4 
horse-power-year,  per  ton  of  pig-iron,  depending  on  the  richness  of 
the  ore.  The  fuel  used  for  reducing  and  carburizing  the  iron  is  600 
or  800  Ib.  of  coke  or  charcoal,  which  need  not  be  of  good  quality. 

Comparing  the  cost  of  smelting  by  the  two  processes,  apart 
from  considerations  of  the  scale  of  working,  which  will  at  first  greatly 
hamper  any  electric-smelting  project,  the  main  items  of  cost  to 
compare  are  the  fuel  and  the  electric  power.  Thus  in  the  electric 
furnace  the  ton  of  pig-iron  would  require,  at  present,  1/4  horse- 
power-year, and  600  or  800  Ib.  of  coke  or  charcoal,  while  the  blast- 
furnace would  require  some  1,900  or  2,000  Ib.  of  coke  for  pure  and 
easily  reducible  ores,  and  as  much  as  2,500  Ib.  or  3,000  Ib.  when  poor 
ores  and  coke  are  used.  Balancing  the  cost  of  1/4  horse-power-year 
against  the  cost  of  the  coke  that  is  saved,  will  give  a  general  idea  of 
the  prices  of  coke  and  power  which  would  permit  of  electric  smelting. 
Of  the  other  expenses  of  the  two  methods,  the  electric  furnace,  receiv- 
ing high-voltage  current  at  a  certain  price,  would  require  transfor- 
mers and  heavy  cables  from  these  to  the  furnace.  The  carbon  elec- 
trodes must  also  be  supplied.  The  blast-furnace,  on  the  other  hand, 

1  P.  L.  T.  Heroult.  Apparatus  for  smelting  iron  ore.  U.  S.  patent  815,293, 
March,  1906.  Electrochemical  Industry,  vol.  iv,  1906,  p.  152. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  197 

has  the  expense  of  the  blowing  engines  with  their  attendant  boilers, 
and  of  the  enormous  hot-blast  stoves  for  preheating  the  blast. 

RECENT  DEVELOPMENTS  IN  ELECTRICAL  IRON  SMELTING 

The  greatest  progress  in  this  direction  has  been  made  in  Sweden, 
where  conditions  are  most  favorable  for  the  commercial  develop- 
ment of  this  process.  Three  Swedish  Engineers,  Messrs.  Gronwall, 
Lindblad,  and  Stalhane,  were  impressed  by  the  experiments  carried 
out  by  Dr.  Haanel  at  Sault  Ste.  Marie  in  1906,  and  decided  to 
devote  their  energies  to  the  production  of  a  furnace  for  the  com- 
mercial operation  of  this  process  in  Sweden.1 

In  view  of  the  fact  that  iron  smelting  in  Sweden  is  done  by  means 
of  charcoal,  and  that  water-powers  are  abundant  and  cheaply  de- 
veloped in  that  country,  the  electrical  smelting  of  iron-ores  is  a 
matter  of  great  importance  to  the  iron  masters  there,  who  accord- 
ingly assisted  very  materially  the  efforts  of  these  three  engineers. 
More  than  $100,000  was  spent  during  two  years  in  developing  a 
furnace  which  should  meet  the  requirements.  Seven  different 
furnaces  were  tried,2  four  of  which  are  illustrated  here. 

In  the  furnace  used  at  Sault  Ste.  Marie  the  carbon  electrode 
passed  down  the  shaft,  and  this  construction  would  obviously  be 
impossible  in  furnaces  of  any  considerable  size.  The  main  point 
to  be  determined  was  how  to  introduce  the  electrode,  and  in  the 
earlier  furnaces  shown  in  Figs.  86  and  87  attempts  were  made  to 
dispense  altogether  with  carbon  electrodes,  employing  in  their 
stead  electrodes  of  molten  pig-iron  similar  in  principle  to  those  of 
the  de  Laval  furnace,  Fig.  20. 

In  the  furnace  shown  in  Fig.  86,  tuyeres  were  employed  as  in 
ordinary  blast-furnace  practice,  it  being  the  intention  to  start  the 
furnace  like  an  ordinary  blast-furnace  and  when  in  regular  operation 
to  switch  on  the  current  and  run  it  electrically.  The  top  of  the 
furnace  was  provided  with  a  cup  and  cone  for  charging,  and  an  off- 
take for  the  gases  as  in  usual  practice.  The  electrodes  consist  of 
two  parallel  channels  AB,  containing  molten  pig-iron.  These  chan- 
nels extend  under  the  shaft  of  the  furnace  and  pass  out  into  pockets, 
CD,  lined  with  carbon  by  means  of  which  electrical  connection  is 

1  Haanel,  Report  on  Electric  Shaft  Furnace,  Domnarfvet,  Sweden,  Ottawa, 
1909. 

2  Haanel,  Bull.  No.  3,  Recent  Advances  in  the  Construction  of  Electric  Fur- 
naces, Ottawa,  1910. 


198 


THE  ELECTRIC  FURNACE 


made  to  the  molten  iron.  Within  the  furnace  the  electric  current 
passes  from  one  channel  to  the  other  through  the  slag  and  melting 
charge,  thus  producing  the  heat  for  running  the  furnace.  Between 
these  channels  is  a  third  channel,  E}  for  tapping  the  iron  and  slag 
from  the  furnace,  and  it  appears  that  molten  iron  will  flow  from  the 
side  channels  (when  these  are  full)  to  the  central  channel,  thus 


FIG.  86. — Domnarfvet  furnace  No.  i.     FIG.  87. — Domnarfvet  furnace  No.  2. 

partially  short-circuiting  the  furnace.  The  hearth  of  the  furnace 
was  lined  with  silicious  material,  5,  and  as  might  have  been  expected, 
this  lining  did  not  last  long,  due  no  doubt  to  the  excessive  flow  of 
current  close  to  the  lining.  This  failure  of  the  lining  would  lead  to 
short  circuits  and  thus  put  an  end  to  the  operation  of  the  furnace. 
In  Fig.  87  the  hearth  was  lined  with  magnesite,  M,  which  was  ex- 
pected to  be  more  refractory  than  the  silicious  lining,  and  the  molten 


PIG-IRON  IN  THE  ELECTRIC  FURNACE 


199 


iron  electrodes  were  led  in  at  opposite  points  of  the  furnace  so  as  to 
avoid  as  far  as  possible  the  danger  of  short-circuiting.  The  same 
defects  were  bound  to  occur  however  in  this  case  and  the  furnace 
had  to  be  given  up. 

With  reference  to  these  furnaces  using  molten  iron  as  electrodes, 
it  may  be  pointed  out  that  the  central  groove  for  tapping  out  the 
metal  appears  to  be  an  unnecessary  source  of  weakness  and  that 


FIG.  88. — Domnarfvet  furnace  No.  3.      FIG.  89. — Domnarfvet  furnace  No.  4. 


the  original  design  of  the  de  Laval  furnace,  Fig.  20,  in  which  the 
molten  metal  is  withdrawn  from  the  electrode  channels,  is  more 
favorable  in  view  of  the  need  of  avoiding  short  circuits. 

After  trying  molten  iron  electrodes,  a  furnace  was  designed,  which 
is  shown  in  Fig.  88,  having  three  graphite  electrodes.  One  of  these 
was  in  the  bottom  of  the  furnace  and  was  covered  and  protected 
by  the  molten  iron;  the  other  two  were  placed  at  the  sides  of  the 


200  THE  ELECTRIC  FURNACE 

furnace  and  were  in  contact  with  the  melting  ore.  The  electric 
current  would  pass  from  the  upper  electrodes  to  the  bottom  electrode, 
and  if  desired  could  be  made  to  pass  between  the  two  lateral  elec- 
trodes. The  furnace  worked  satisfactorily  for  a  time  but  the 
brick  work  around  the  upper  electrodes  became  corroded,  due 
to  the  high  temperature  produced  at  these  points.  The  experi- 
ence gained  with  this  furnace  showed  that  the  electrode  should  not 
be  in  contact  with  the  furnace  wall  and  the  ore  at  the  same  point. 
The  next  design  of  furnace  was  arranged  to  avoid  this  contact. 
This  furnace,  shown  in  Fig.  89,  consists  of  a  somewhat  narrow  shaft, 
S,  and  a  wider  hearth,  H,  into  which  the  shaft  enters.  The  elec- 
trodes, of  which  there  are  three,  enter  through  the  arched  roof  of 
the  hearth,  and  the  ore  lying  at  its  natural  angle  of  repose  does  not 
reach  the  roof  of  the  furnace  at  the  points  where  the  electrodes  enter. 

As  the  result  of  the  preliminary  experiments  the  furnace  shown 
in  Fig.  89  was  adopted  as  the  most  suitable,  and  in  the  year  1908  a 
larger  furnace  of  the  same  general  pattern  was  erected.1  A  section 
of  this  furnace  is  shown  in  Fig.  90. 2 

Domnarfvet  Furnace. — The  furnace  consists  of  a  circular  smelting 
chamber  7  ft.  6  in.  in  internal  diameter  and  5  ft.  high,  surmounted 
by  a  shaft  17  ft.  high  and  5  ft.  in  maximum  diameter.  This  shaft 
is  reduced  at  its  lower  end  to  about  3  ft.  in  order  that  the  ore  pass- 
ing through  this  restricted  opening  and  lying  within  a  cone  shown 
by  the  dotted  lines  shall  not  come  in  contact  with  the  roof  of  the 
smelting  chamber  at  the  point  where  electrodes  enter.  The  smelt- 
ing chamber  or  hearth  of  the  furnace  is  constructed  of  fire-brick  and 
has  a  lining  of  magnesite.  It  is  provided  with  a  single  tapping-hole. 
The  shaft  of  the  furnace  is  constructed  of  fire-brick  and  is  supported 
on  steel  struts  so  that  the  weight  does  not  press  upon  the  roof  of  the 
smelting  chamber.  The  ore  and  fuel  are  charged  through  a  cup  and 
cone  of  special  design,  the  Tholander  charging  bell,  which  is  com- 
mon in  Swedish  blast-furnaces.  This  is  designed  to  allow  of  the 
charcoal  being  delivered  to  the  middle  of  the  furnace  and  the  ore 
to  the  sides.  A  movable  hood  covers  the  charging  apparatus  and 
serves  to  prevent  the  escape  of  gases  or  the  entrance  of  air  during 
charging.  There  are  two  gas  off-takes,  one  of  which  permits  the 
gas  to  escape  outside  the  building,  and  the  other  communicates  to 
a  dust-catcher  and  from  that  to  a  fan  which  serves  to  blow  a  portion 

1  Haanel,  loc.  cit. 

2  Reproduced  from  illustration  in  paper  by  Dr.  Haanel,  Trans.  Am.  Electro- 
them.  Soc.,  xv,  1909,  p.  26. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE 


201 


of  the  gases  into  the  smelting  chamber  through  three  tuyeres  enter- 
ing just  beneath  the  arch  of  the  furnace.  The  furnace  is  provided 
with  three  electrodes  entering  through  the  arched  roof  and  supplied 
with  three-phase  current.  The  electrodes  enter  through  water- 
cooled  stuffing-boxes,  which  can  be  kept  tight  by  means  of  asbestos 
packing.  The  electrodes  themselves  are  n  in.  X  22  in.  in  cross- 
section  and  as  electrodes  of  this  size  were  not  available  they  were 


ffffft 

\  Ordinary  Brick        ¥%%  Fire  Brick 

FIG.  90. — Domnarfvet  furnace,  final  form. 

made  by  clamping  together  two  electrodes  n  in.  square.  The 
electrodes  are  held  at  their  upper  ends  by  clamping  pieces  which 
serve  to  lead  in  the  electric  current,  and  also  to  adjust  the  height  of 
the  electrode  by  means  of  a  wire  rope  and  winch.  The  electrode 
holder  glides  between  guides.  In  operating  a  furnace  of  this  design 
the  gases  produced  in  the  smelting  chamber  are  not  sufficient  in 
amount  to  heat  the  charge  in  the  shaft  to  a  temperature  sufficiently 


202  THE  ELECTRIC  FURNACE 

\ 

high  for  the  reduction  of  the  ore  by  carbon  monoxide.  The  circula- 
tion system  described  above  overcomes  this  difficulty  by  causing 
a  portion  of  the  gases  from  the  top  of  the  furnace  to  enter  through 
the  tuyeres  and  to  pass  up  the  shaft  again.  This  circulation  has 
the  effect  of  conveying  heat  from  the  smelting  chamber  up  the  shaft 
where  it  serves  to  preheat  the  charge.  The  gases  entering  beneath 
the  arch  of  the  furnace  also  serve  to  cool  this  arch  and  prevent  its 
becoming  overheated  or  melted,  and  in  this  way  the  heat  transferred 
to  the  shaft  of  the  furnace  is  withdrawn  from  a  point  where  it  can 
be  very  well  spared.  The  tuyeres  are  introduced  midway  between 
adjacent  electrodes. 

The  electrical  supply  for  this  furnace  was  arranged  to  give  great 
flexibility  in  operation  and  is  different  from  what  would  be  adopted 
in  regular  practice.  It  consists  of  a  three-phase  synchronous  motor 
of  900  h.p.  supplied  with  6o-cycle  current  at  7,000  volts.  A  three- 
phase  generator,  directly  coupled  to  the  motor,  supplies  25-cycle 
current  at  a  voltage  which  is  adjustable  by  small  steps  from  300  to 
1,200  volts.  The  current  from  the  genera  tor  is  taken  to  transformers, 
placed  close  to  the  furnace,  by  which  the  voltage  is  lowered  in  the 
ratio  of  14  to  i;  the  secondary  voltage  ranging  from  20  to  80  volts. 
In  order  that  the  full  power  of  the  plant  (500  or  600  kw.)  may  be 
available  throughout  this  range  of  voltage  the  transformers  have  a 
total  capacity  of  1,500  kilo-volt-amperes. 

Results  Obtained. — The  furnace  was  erected  in  the  year  1908  and 
a  trial  run  was  made  at  the  end  of  that  year.  The  run  extended 
from  the  yth  of  May  to  the  3Oth  of  July,  1909,  and  demonstrated 
that  the  design  was  satisfactory  for  regular  operation  besides  giving 
valuable  information  with  regard  to  the  economy  of  the  process. 
The  ores  smelted  during  this  time  were  a  number  of  Swedish  magne- 
tites. The  fuel  used  was  charcoal  and  mixtures  of  charcoal  and  coke, 
and  the  consumption  of  fuel  varied  from  0.30  to  0.39  tons  per  ton 
of  pig-iron.  The  output  of  pig-iron  was  quite  small,  being  about 
4  tons  daily,  and  the  power  consumption  was  somewhat  large,  aver- 
aging 2  tons  of  pig-iron  per  kilowatt-year.  A  better  figure  may  be 
expected  in  the  future.  The  consumption  of  electrodes  was  also 
somewhat  large  in  this  furnace,  averaging  about  30  Ib.  per  ton  of 
pig-iron.  The  power-factor  of  the  furnace  running  at  25  cycles 
varied  from  0.8  to  0.9  and  may,  therefore,  be  considered  quite 
satisfactory. 

In  regard  to  the  circulation  of  the  gases,  and  the  economy  of  the 
furnace,  it  is  important  to  know  the  analysis  of  the  gases  leaving 


PIG-IRON  IN  TEE  ELECTRIC  FURNACE  203 

the  top  of  the  furnace.  These  gases  consist  almost  entirely  of  car- 
bon monoxide  and  carbon  dioxide.  When  the  furnace  is  operated 
without  circulation  the  top  of  the  charge  is  quite  cool,  below  100° 
C.,  and  the  gases  only  contain  about  10  per  cent,  of  carbon  dioxide. 
When  the  circulation  is  maintained  the  temperature  of  the  furnace 
top  rises  to  200°  C.  or  300°  C.  and  the  percentage  of  carbon  dioxide 
increases  to  about  30  per  cent,  or  40  per  cent.  It  should  be  borne 
in  mind  that  although  a  part  of  the  gas  is  returned  to  the  furnace 
this  does  not  imply  any  diminution  in  the  amount  escaping.  The 
economy  of  the  furnace  depends  very  largely  on  the  analysis  of  the 
escaping  gases,  the  efficiency  being  greater  both  in  regard  to  fuel 
consumption  and  electrical  power  consumption  when  the  proportion 
of  carbon  dioxide  is  high.  It  will  be  seen,  therefore,  that  the  cir- 
culation of  the  gases  has  a  very  important  effect  on  the  operation 
of  the  furnace. 

Frick  Electric  Reduction  Furnace. — The  Frick  furnace,1  shown 
in  Fig.  91,  has  been  designed  for  the  purpose  of  economizing  elec- 
trodes. This  is  effected  by  the  use  of  coke  which  is  fed  in  around 
each  electrode  so  as  to  form  the  working  electrode.  The  electrode 
itself  is  vertical,  and  being  surrounded  by  coke,  its  consumption 
should  be  greatly  reduced.  Otherwise  the  design  and  construction 
of  this  furnace  is  substantially  the  same  as  in  the  Domnarfvet 
furnace  already  described. 

Calif ornian  Furnaces. — California  offers  a  location  favorable  for  the 
commercial  development  of  the  electric  smelting  of  iron-ores.  This 
is  mainly  on  account  of  the  high  market  price  of  pig-iron  in  that  state. 
There  are  good  water-powers  for  operating  such  furnaces  and  char- 
coal can  be  obtained  at  a  reasonable  figure.  Conditions  in  California 
are  not  as  favorable  as  in  Sweden  for  the  cheap  production  of  electric 
pig-iron,  but  in  view  of  the  higher  price  of  pig-iron  in  California  a 
commercial  industry  may  very  probably  be  founded  there.  In 
1907  a  2,ooo-h.p.  furnace  was  erected  by  Paul  Heroult  at  a  place 
named  Heroult  on  the  Pitt  River  in  Shasta  Co.2  This  furnace 
which  was  expected  to  give  20  tons  of  pig-iron  daily  was  started  on 
the  4th  of  July,  1907,  but  did  not  prove  satisfactory  and  was  ulti- 

1  Eugene  Haanel,  Ph.  D.,  Director  of  Mines,  "Recent  Advances  in  the  Con- 
struction of  Electric  Furnaces  for  the  Production  of  Pig-iron,  Steel  and  Zinc." 
Bull.  No.  3,  Ottawa,  1910. 

2  Electrochemical  and  Metallurgical  Industry,  vol.  v,  1907,  p.  318;  Haanel 
Report  on  Experiments  at  Sault  Ste.  Marie,  Ottawa,  1907.     See  also  reference 
to  this  furnace  on  page  189. 


204 


THE  ELECTRIC  FURNACE 


mately  torn  down.  The  furnace1  consisted  of  a  long  elliptical- 
shaped  iron  box  with  refractory  brick  lining  around  the  sides,  and 
a  carbon  lining  on  the  bottom,  as  in  the  simple  Heroult  furnace, 
Fig.  78,  but  provided  with  a  roof.  This  formed  a  smelting  chamber 


FIG.  91. — Frick  iron-smelting  furnace. 

having  three  vertical  carbon  electrodes,  each  connected  to  one  pole 
of  a  i,5oo-kw.  three-phase  supply;  the  bottom  of  the  furnace  being 
connected  to  the  neutral  point  as  in  Fig.  84.  The  electrodes  were 
arranged  in  a  straight  line  (the  major  axis  of  the  furnace),  and  alter- 

1  D.  A.  Lyon,  "The  Electric  Furnace  in  the  Production  of  Iron  from  Ore." 
Met.  and  Chem.  Engng.,  xi,  1913,  p.  16. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  205 

nated  with  four  vertical  chutes  for  supplying  the  ore  charge  to  the 
furnace.  In  order  to  preheat  the  charge,  the  chutes  were  heated  by 
burning  around  them  the  gases  liberated  from  the  furnace.  The 
chutes  became  choked  with  the  heated  ore,  and  even  the  roof  could 
not  be  maintained  in  operation,  so  the  furnace  could  only  be  employed 
with  an  open  top  and  was  finally  given  up. 

As  this  furnace  was  not  satisfactory,  experiments  were  made  under 
the  direction  of  Prof.  Dorsey  A.  Lyon  of  Stamford  University.1 
In  May,  1908,  he  had  constructed  a  single-phase  furnace  using  160 
kw.  and  having  an  output  of  i  ton  of  pig-iron  daily.  He  then  con- 
structed, in  1909,  a  furnace  of  1,500  kw.  This  furnace  was  substan- 
tially the  same  as  the  Domnarfvet  furnace.  There  was,  however, 
no  circulation  of  the  furnace  gases,  and  holes  were  provided  by  which 
air  entered  the  shaft  above  the  level  of  the  charge.  The  air  served 
to  burn  a  portion  of  the  gases  within  the  shaft,  thus  heating  the  charge, 
and  the  escaping  gases  were  led  to  chambers  where  the  ore  was  pre- 
heated. In  this  way  the  ore  entered  the  furnace  at  a  fair  tempera- 
ture and  there  was  not  the  same  need,  as  in  the  Domnarfvet  furnace, 
of  circulating  the  gases  for  heating  the  charge  in  the  shaft.  Not 
very  much  has  been  published  with  regard  to  the  operation  of  this 
furnace  but  the  author  understands  that  the  electrodes  gave  trouble 
by  breaking,  and  that  it  was  found  to  be  necessary,  on  this  account, 
to  employ  the  Acheson  graphite  electrodes.  At  the  present  time 
it  is  probable  that  carbon  electrodes  of  sufficient  size  and  strength 
can  be  obtained,  and  these  can  be  furnished  with  threaded  ends 
so  that  fresh  lengths  can  be  screwed  on  to  the  working  electrodes 
whenever  they  become  too  short,  thus  affording  a  continuous  feed 
and  avoiding  any  waste  of  the  stub  ends. 

The  electrical  equipment  for  the  Lyon  furnace  consisted  of  three 
transformers  of  750  kw.  each,  supplied  with  6o-cycle  current  at 
2,200  volts.  The  voltage  of  the  current  passing  to  the  furnace 
varied  from  35  to  75  volts.  The  regulation  was  effected  by  means 
of  taps  in  the  primary  windings;  these  being  arranged  to  alter  the 
secondary  voltage  in  steps  of  three  volts.  In  this  furnace,  as  in 
other  recent  furnaces,  there  is  no  need  to  raise  and  lower  the  electrodes 
for  the  purpose  of  regulating  the  current.  The  electrodes  are  only 
moved  occasionally  as  they  wear  away;  the  regulation  of  the  current 
being  effected  by  changing  the  voltage  of  the  supply. 

The  Noble  Electric  Steel   Company  have  recently   developed 

1  D.  A.  Lyon,  Am.  Electrochem.  Soc.,  vol.  xv,  1909,  p.  39. 


206  THE  ELECTRIC  FURNACE 

another  type  of  furnace,1  which  is  now  in  commercial  operation  in 
California.  This  furnace  has  a  general  resemblance  to  the  one 
constructed  there  by  Dr.  Heroult.  It  consists  of  a  steel  shell  27  ft. 
long,  13  ft.  wide  and  12  ft.  high,  lined  with  refractory  material, 
and  provided  with  an  arched  roof  and  a  tapping  hole.  The  ore 
charge  enters  through  five  vertical  chutes,  and  the  electric  current 
by  four  vertical  electrodes  which  alternate  with  the  chutes,  and  all 
enter  the  roof  at  points  on  its  longer  diameter.  The  chutes  are 
2  ft.  in  internal  diameter,  and  15  ft.  high.  They  are  used  merely 
for  charging,  no  preheating  being  attempted.  The  electrodes  are 
of  graphite,  12  in.  in  diameter,  and  4  ft.  long,  new  sections  being 
attached  with  a  screwed  connection  (see  Fig.  48).  Each  electrode 
lasts  for  30  days  in  continuous  operation.  The  electrode- jackets 
and  the  arched  roof  are  water-cooled.  Three  75o-kw.  transformers 
are  connected  to  the  2, 400- volt  three-phase  supply,  and  deliver  cur- 
rent to  the  electrodes  at  40  to  80  volts.  The  voltage  regulation  is 
effected  by  means  of  eight  current  taps  and  a  compensator  on  the 
primary  side  which  gives  fifteen  steps  for  voltage  variation.  Appar- 
ently (as  there  are  four  electrodes)  the  low-tension  terminals  of  each 
transformer  will  be  connected  to  each  adjacent  pair  of  electrodes  so 
that  each  section  of  the  furnace,  between  two  electrodes,  will  receive 
the  power  from  one  transformer;  the  two  end  electrodes  will,  how- 
ever, carry  a  smaller  current  than  the  other  two. 

The  ore  is  a  very  pure  magnetite  containing  69.9  per  cent,  of  iron 
and  2.4  per  cent,  of  silica;  it  is  smelted  in  admixture  with  charcoal, 
quartz  and  burned  lime,  a  typical  charge  being: 

Iron-ore 500  Ib. 

Charcoal 135  to  150  Ib. 

Lime 3^  Ib. 

Quartz 12%  Ib. 

In  the  operation  of  this  furnace  no  use  is  made  of  the  liberated  gases, 
either  for  the  preheating  or  the  reduction  of  the  ore,  and  the  gases 
are  not  circulated  as  in  the  Swedish  furnaces. 

Since  the  above  was  printed,  an  illustrated  account  of  the  Cali- 
fornian  plant  has  appeared,2  in  which  it  is  stated  that  a  3,000  K.  W. 
furnace  of  the  same  type  has  now  been  constructed.  The  power 
consumption  has  been,  as  low  as  2,200  K.W.  hours  (0.337  H.P. 

1  D.  A.  Lyon,  "The  Electric  Furnace  in  the  Production  of  Iron  from  Ore," 
Met.  and  Chem.  Eng.,  xi,  1913,  p.  17. 

2  J.  Crawford,  "Progress  of  Electric  Smelting  at  Heroult,  California,"  Met.  and 
Chem.  Eng.,  xi,  1913,  p.  383. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  207 

years)  per  ton  of  pig  iron,  when  using  3,000  K.W.  The  charcoal 
used  for  reduction  (for  a  pig  of  1.5  per  cent,  silicon)  carries  fixed 
carbon  equal  to  about  26  per  cent,  of  the  ore,  which  will  be  about 
0.40  ton  of  charcoal  per  ton  of  pig.  The  charcoal  is  calculated  on 
the  assumption  that  the  ore  is  reduced  entirely  by  carbon,  and  not 
at  all  by  CO.  The  furnace  gases  contain  about  62  per  cent,  of  CO 
and  7  per  cent,  of  CO2.  The  furnace  is  admitted  to  be  less  efficient 
than  the  Swedish  furnace,  but  it  is  more  easily  operated,  and  a 
saving  is  effected  by  using  the  gases  for  heating  lime-kilns  and 
charcoal-retorts. 

Helfenstein  Furnace. — This  furnace  was  devised  for  the  produc- 
tion of  calcium  carbide  and  ferro-silicon,  and  is  described  under 
these  headings.  It  resembles  the  Californian  furnace  in  principle 
and  has  recently  been  applied  to  the  smelting  of  iron-ores.  A 
furnace  with  six  shafts  uses  24,000  h.p.  and  has  an  output  of  250 
tons  of  pig  iron  per  day.1 

Trollhattan  Furnace. — After  the  experience  gained  with  the  700- 
h.p.  furnace  at  Domnarfvet  the  " Aktiebolaget  Elektrometall " 
proceeded  to  construct  a  larger  furnace  of  2,500  h.p.  at  Trollhattan.2 
In  this  undertaking  they  were  assisted  by  the  Swedish  Government 
who  furnished  them  with  power  at  a  nominal  figure  from  their 
Trollhattan  power  station.  The  furnace  shown  in  Fig.  Q23  resem- 
bles the  Domnarfvet  furnace  in  general  construction,  but  the  shaft 
of  the  furnace  is  supported  in  a  different  manner  and  the  furnace  is 
provided  with  four  electrodes  instead  of  three.  The  furnace  is  45  ft. 
high,  the  hearth  is  12  ft.  6  in.  in  internal  diameter  and  7  ft.  high. 
The  shaft  is  7  ft.  6  in.  in  internal  diameter  and  is  reduced  to  4  ft.  at 
the  point  where  it  enters  the  hearth.  The  four  electrodes  were 
each  26  in.  square,  but  more  recently  round  electrodes,  24  in.  in 
diameter,  have  been  used.  The  hearth  is  built  in  a  steel  shell,  it  is 
lined  with  fire-brick  and  has  an  inner  lining  of  magnesite-brick. 
The  basin-shaped  bottom  of  the  furnace  has  a  rammed  lining  of 
magnesite  and  tar  which  extends  nearly  to  the  top  of  the  walls.  The 
roof  of  the  furnace  is  a  fire-brick  dome.  The  shaft  is  built  in  a  steel 
shell  which  is  hung  by  means  of  an  octagonal  ring  from  two  steel 
beams  which  are  supported  on  the  walls  of  the  furnace  room.  The 
shaft  is  lined  with  fire-brick  and  is  provided  with  numerous  holes  for 

1Met.  and  Chem.  Eng.  x,  1912,  p.  686. 

2  T.  D.  Robertson,  "Recent  Progress  in  Electrical  Iron-Smelting  in  Sweden," 
Trans.  Am.  Electrochem.  Soc.,  vol.  xx,  1911,  p.  375. 

3  Reproduced  from  Mr.  Robertson's  paper. 


208 


THE  ELECTRIC  FURNACE 


the  insertion  of  pyrometers;  the  whole  plant  being  provided  with 
every  appliance  for  the  exact  observation  of  the  operation  of  the 
furnace.  The  output  of  the  furnace  is  about  23  tons  of  pig-iron 
daily. 


'ing  Floor 


FIG.  92. — Trollhattan  furnace. 

Electrical  Supply. — The  electrical  power  is  taken  from  the  Govern- 
ment power  station  which  furnishes  three-phase  25-cycle  current  at 
1 0,000  volts.  The  furnace  is  provided  with  four  electrodes  to  operate 
with  two-phase  current;  no  doubt  with  the  expectation  of  obtaining 
more  uniform  heating  than  if  three-phase  current  were  employed. 
The  three-phase  high-voltage  supply  is  changed  into  two-phase 
current  at  a  low  voltage  by  means  of  two  transformers  arranged  with 
the  Scott  connections.  The  arrangement  is  shown  in  Fig.  53.  In 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  209 

order  to  provide  a  considerable  range  of  voltage  in  the  furnace  sup- 
ply, the  connections  to  the  primary  windings  of  the  transformers 
can  be  changed  by  steps  so  as  to  vary  the  voltage  of  the  secondary 
winding  from  50  to  90  volts.  The  transformers  supply,  to  the 
electrodes,  a  current  of  from  12,000  to  22,00x3  amperes  on  each  phase. 
Results  Obtained. — The  furnace  was  started  in  November,  1910. 
The  first  report  of  this  furnace1  was  made  after  it  had  been  in  opera- 
tion for  about  six  months  and  the  behavior  of  the  furnace  had  been 
entirely  satisfactory  during  that  time.  The  efficiency  had  been 
better  than  that  of  the  Domnarfvet  furnace  and  showed  a  produc- 
tion of  pig-iron  as  large  as  three  tons  per  horse-power-year,  although 
the  average  was  somewhat  less  than  that.  The  amount  of  fuel 
employed  was  about  1/3  ton  per  ton  of  pig-iron  and  the  net  con- 
sumption of  electrodes  was  about  10  Ib.  per  ton  of  pig-iron.  The 
thermal  efficiency  was  stated  to  be  80  per  cent.  The  average 
output  of  the  furnace  was  13  tons  per  day  with  an  average  power 
of  1,344  kw.  More  recent  information  shows  a  better  efficiency 
than  was  obtained  at  that  time,  the  best  production  being  as  much 
as  4  tons  of  pig-iron  per  horse-power-year,  which  is  very  nearly 
as  large  as  one  would  expect  such  a  furnace  to  make.  The  earlier 
electrodes  were  made  by  clamping  together  four  small  electrodes 
13  in.  square,  and  with  these  electrodes  there  was  a  considerable 
waste  due  to  the  unused  ends  when  the  electrodes  became  too  short 
for  further  use.  Using  round  electrodes  with  threaded  joints, 
this  waste  of  short  ends  is  avoided  as  the  electrodes  are  fed  continu- 
ously into  the  furnace.  With  the  old  electrodes,  moreover,  the 
electrical  connection  was  made  through  the  holder  at  the  head  of 
the  electrode,  while  with  the  round  electrodes  the  electric  current 
is  led  in  through  holders  which  are  close  to  the  arch  of  the  furnace. 
This  arrangement  saves  a  considerable  amount  of  power  and  inci- 
dentally allows  of  the  furnace  being  supplied  with  3,000  h.p. 
instead  of  2,500  as  was  originally  expected.  The  furnace  is  provided 
like  the  Domnarfvet  furnace  with  an  arrangement  for  the  circula- 
tion of  the  gases.  More  elaborate  provision  has  been  made  for 
removing  the  dust  by  means  of  dust-catchers  and  water-scrubbers, 
because  the  dust  in  these  gases  was  harmful  to  the  fans  used  for 
circulation.  In  the  equipment  of  the  present  furnace  a  reserve 
fan  has  been  installed  to  permit  of  repairs.  In  connection  with  the 
gas  circulation  it  is  of  interest  to  observe  that  this  has  a  bad  effect 
on  the  electrodes;  the  gases  returned  to  the  hearth  contain  a  notable 

1  T.  D.  Robertson,  loc.  cit. 
14 


210  THE  ELECTRIC  FURNACE 

proportion,  as  much  as  30  per  cent.,  of  carbon  dioxide,  and  this 
oxidizes  and  corrodes  the  carbon  electrodes  between  the  roof  and 
the  charge  in  the  furnace.  This  corrosion  necessitates  a  somewhat 
more  frequent  feeding  of  the  electrodes  which  might  otherwise  be 
completely  cut  in  two. 

At  the  time  of  this  report  the  following  furnaces  were  in  operation 
or  in  course  of  erection  in  Norway  and  Sweden: 

Sweden  Trollhatten,  i  furnace 3,000  h.p. 

Sweden  Domnarfvet,  i  furnace 4,000  h.p. 

Sweden  Hagfors,  2  furnaces  (3,000  h.p.  each) 6,000  h.p. 

Norway  Tyssedahl   (Hardanger),  2   furnaces  (3,500  h.p. 

each) 7,000  h.p. 

Norway  Arendal,  2  furnaces  (2,500  h.p.  each) 5,000  h.p. 

25,000  h.p. 

In  newer  furnaces  three-phase  current  and  six  electrodes  are 
employed,  and  furnaces  of  5,000  h.p.  and  even  7,500  h.p.  are  now 
in  course  of  construction. 

A  report  of  the  Trollhatten  furnace  has  been  made  by  A.  Leffler 
and  E.  Nystrom,1  and  covers  the  period  from  August  4,  1911,  to 
March  6,  1912.  During  this  period  of  215  days,  3,214  metric  tons 
of  iron  were  produced,  that  is  15  tons  daily,  with  an  average  power 
of  1,482  kw.  The  furnace  was  inactive  during  7  per  cent,  of  the 
time,  which  would  indicate  an  output  of  16  tons  daily  during  regu- 
lar operation.  The  consumption  of  electrical  energy  was  2,225 
kw.-hours,  or  0.34  E.H.P.  year  per  ton  of  pig-iron.  The  consump- 
tion of  fuel  (charcoal  with  a  little  coke)  was  0.407  ton,  per  ton  of 
pig-iron,  and  the  electrode  consumption  was  5.72  kg.  (12.5  Ib.) 
per  ton  of  pig-iron.  The  circulated  gases  contained  23.5  per  cent. 
CO2,  63  per  cent.  CO,  10.3  per  cent.  H2  and  1.5  per  cent.  N2.  The 
analyses  of  the  pig-iron  show  the  carbon  usually  between  3.3  per 
cent,  and  3.7  per  cent.,  the  sulphur  usually  under  o.oi  per  cent., 
and  phosphorus  usually  between  o.oi  per  cent,  and  0.02  per  cent. 
The  silicon  is  very  variable,  running  for  two  months  from  o.i  per 
cent,  to  0.5  per  cent,  but  at  other  times  as  high  as  6  per  cent. 

ELECTRIC  FURNACE  DESIGN 

The  electric  shaft-furnace  for  the  smelting  of  iron-ores  appears 
to  have  arrived  at  a  fairly  definite  type  in  the  Swedish  furnace.  The 

1  Neumann,  Eisen  und  Stahl.     Iron  Age,  Nov.  28,  1912,  p.  1297. 


PIG-IRON  IN  THE  ELECTRIC  FURNACE  211 

special  features  of  this  furnace  are  the  enlarged  hearth,  allowing  of 
the  entrance  of  electrodes,  and  the  circulation  of  the  gases  for  heat- 
ing the  charge  in  the  shaft.  With  regard  to  these  features  it  may  be 
pointed  out  firstly  that  a  shaft  furnace  can  be  constructed  either  after 
this  pattern  or  with  a  central  electrode,  or  group  of  electrodes,  smelt- 
ing ore  furnished  by  a  number  of  lateral  shafts.  This  latter  type 
is  suggested  in  the  Haanel-Heroult  furnace,  Fig.  81.  The  introduc- 
tion in  such  a  furnace  of  the  ore  from  the  sides  would  appear  to  favor 
the  economy  of  heat  in  the  smelting  region,  but  on  the  other  hand  the 
maintenance  of  the  roof  in  such  a  furnace  must  be  more  difficult,  and 
it  seems  probable  that  except  for  special  purposes  the  Swedish  would 
be  found  the  better. 

With  regard  to  the  circulation  of  the  gases,  while  these  serve  the 
useful  purpose  of  heating  the  charge  and  of  cooling  the  arch  of  the 
furnace,  it  must  be  remembered  that  they  remove  heat  from  the 
smelting  chamber,  where  it  should  be  economized,  and  that  on  ac- 
count of  the  carbon  dioxide  present  they  serve  to  corrode  the  elec- 
trodes and  also  no  doubt  to  consume  some  of  the  carbon  from  the 
charge  in  the  hearth  and  lower  part  of  the  shaft.  The  preheating 
of  the  charge  in  the  shaft  can  be  effected  more  economically  by  means 
of  the  calorific  power  of  the  gases  escaping  from  the  furnace  than  by 
circulating  them  as  in  the  Swedish  furnace.  There  are,  however, 
practical  difficulties  in  the  way  of  carrying  this  out.  One  method  is 
to  heat  the  charge  by  conduction  through  the  walls  of  retorts,  pref- 
erably vertical,  and  this  method  has  been  adopted  in  the  Evans- 
Stansfield  furnace  for  the  direct  production  of  Steel  (see  Fig.  no). 
Another  method  is  that  referred  to  on  page  196,  and  patented  by  Paul 
Heroult,  of  introducing  the  carbon  toward  the  lower  end  of  the  shaft 
and  burning  the  gases  higher  up  in  the  shaft  by  the  introduction  of 
air. 


CHAPTER  VIII 
THE  PRODUCTION  OF  STEEL  FROM  METALLIC  INGREDIENTS 

Although  steel  can  be  produced  directly  from  iron-ore,  it  is  almost 
universally  made  from  pig-iron  or  wrought-iron. 

Tool  steel  is  made  from  wrought-iron  by  melting  it  in  crucibles 
with  sufficient  charcoal  to  produce  the  desired  percentage  of  carbon 
in  the  resulting  steel.  In  Sheffield  practice,  steel  is  made  by  heating 
bars  of  wrought-iron  packed  in  charcoal.  The  resulting  "blister 
steel"  is  then  melted  in  crucibles. 

Steel  is  far  more  commonly  made,  however,  by  removing  some  of 
the  carbon  and  other  impurities  from  pig-iron.  This  is  effected 
either  by  blowing  air  through  the  molten  pig-iron  as  in  the  Bessemer 
converter  or  by  adding  iron-ore  and  steel  scrap  to  the  molten  pig- 
iron  as  in  the  open-hearth  furnace.  In  the  first  case  the  heat  of  the 
reaction  is  sufficient  to  keep  the  metal  melted  during  the  operation, 
but  in  the  second  case  the  furnace  is  heated  by  means  of  gas.  In 
making  steel  by  the  Bessemer  process  the  carbon  is  completely  re- 
moved from  the  pig-iron  and  the  necessary  amount  of  carbon,  man- 
ganese, etc.,  is  added  in  the  ladle  at  the  end  of  the  operation.  In  the 
open-hearth  process  it  is  sometimes  possible  to  stop  the  operation 
when  the  metal  contains  the  desired  amount  of  carbon,  but  it  is 
frequently  necessary  to  remove  nearly  the  whole  of  the  carbon  and 
then  to  add  the  required  amount.  In  making  steel  by  either  proc- 
ess the  elimination  of  sulphur  and  phosphorus  has  to  be  considered. 
Some  varieties  of  pig-iron  are  sufficiently  free  from  these  elements, 
and  then  the  operation  can  be  carried  out  in  a  furnace  having  a  sili- 
ceous lining;  silica  being  the  cheapest  and  most  satisfactory  material 
available.  When  using  varieties  of  pig  iron  which  contain  more 
phosphorus  than  is  permissible  in  the  resulting  steel,  it  is  necessary 
to  use  a  furnace,  whether  the  Bessemer  converter,  or  the  open-hearth 
furnace,  which  is  lined  with  a  basic  material  such  as  dolomite  or  mag- 
nesite.  In  such  a  furnace  the  phosphorus  can  be  eliminated  in  the 
presence  of  a  slag  very  rich  in  lime,  and  it  is  also  possible  to  remove 
any  excess  of  sulphur  from  the  steel,  although  the  pig-iron  employed 
should  be,  in  any  case,  nearly  free  from  this  substance. 

212 


STEEL  FROM  METALLIC  INGREDIENTS  213 

In  open-hearth  practice  the  materials  charged  are  pig-iron,  steel 
scrap,  and  iron-ore,  together  with  lime  and  other  materials  needed 
for  effecting  the  refining  of  the  steel.  Pig-iron  may  be  charged  cold, 
but  in  modern  practice  it  is  frequently  brought  from  the  blast- 
furnace to  the  open-hearth  furnace  in  a  molten  condition,  thus  saving 
the  expense  of  molding  and  remelting. 

THE  ELECTRICAL  PRODUCTION  OF  STEEL 

The  electrical  production  of-  steel  from  metallic  ingredients  is 
accomplished  in  furnaces  which  resemble  in  their  action  the  open- 
hearth  furnace  or  the  crucible  furnace.  Some  of  these,  like  the 
crucible  furnace,  are  used  for  the  fusion  of  suitable  pure  materials 
in  the  proportions  required  for  making  steel;  these  materials  would  be 
pure  varieties  of  pig-iron,  and  wrought-iron  or  mild  steel.  Other 
electrical  furnaces,  however,  are  employed  for  melting  together  pig- 
iron,  scrap-steel  and  ore,  as  in  the  open-hearth  furnace,  and  are  almost 
always  made  with  a  basic  lining  so  as  to  allow  of  the  removal  of  phos- 
phorus and  sulphur  from  the  steel  by  means  of  a  limey  slag.  Elec- 
tric furnaces  are  also  employed,  and  this  is  one  of  the  most  important 
of  their  uses,  for  finishing  and  refining  steel  that  has  been  made  in  the 
Bessemer  or  open-hearth  furnace. 

The  furnaces  used  for  steel-making  are  of  three  types:  Arc-furnaces, 
Induction  furnaces  and  Resistance  furnaces.  There  are  two  kinds 
of  arc-furnaces,  the  series- arc  furnace,  Fig.  12,  and  the  single-arc  fur- 
nace, Fig.  ii.  In  the  first  kind,  exemplified  by  the  Heroult  furnace, 
two  carbon  electrodes  enter  through  the  roof  of  the  furnace,  and  the 
electric  current  passes  down  through  one  of  these,  enters  the  slag  and 
metal  in  the  furnace,  and  returns  by  the  other  electrode;  an  electric 
arc  being  maintained  between  the  end  of  each  electrode  and  the  molten 
slag  and  metal.  The  hearth  of  the  furnace  is  made  of  burnt  mag- 
nesite  or  similar  material. 

In  the  second  kind  of  furnace,  typically  the  Girod  furnace,  there 
may  be  only  one  electrode  passing  through  the  roof  of  the  furnace, 
and  the  electric  current,  which  enters  by  this  electrode,  passes  out 
through  the  hearth  of  the  furnace.  For  this  purpose  the  hearth 
must  be  an  electric  conductor,  and  this  is  accomplished,  in  the  Girod 
furnace,  by  inserting  in  the  hearth  a  number  of  vertical  steel  bars 
which  are  in  contact  at  their  upper  ends  with  the  molten  metal  in 
the  furnace,  and  are  water-cooled  at  their  lower  ends  where  they  make 
contact  with  the  electric  conductors. 


214  THE  ELECTRIC  FURNACE 

Induction  furnaces  have  no  electrodes  at  all,  but  the  molten  steel 
is  in  the  form  of  a  ring,  and  an  electric  current  is  induced  in  this  ring 
of  molten  steel,  in  the  same  way  as  in  the  secondary  winding  of  the 
ordinary  electric  transformer. 

Resistance  furnaces  for  making  steel  have  been  constructed  by 
Mr.  Gin  who  used  a  long  folded  channel  containing  the  molten  metal, 
which  is  heated  by  a  large  electric  current  introduced  by  means  of 
water-cooled  electrodes  at  the  two  ends  of  the  channel. 

SERIES-ARC  FURNACES 

The  Heroult  furnace  is  the  best-known  example  of  a  series-arc 
furnace  applied  to  steel-making.  This  furnace  was  described  by 
Dr.  Haanel  in  1904,  and  an  account  will  first  be  given  of  the  small 
furnaces  in  operation  at  that  time. 

The  Heroult  steel  furnace,  Fig.  93, l  resembles  a  Wellman  tilting, 
open-hearth  furnace,  from  which  the  gas  and  air  ports  have  been  re- 
moved, and  with  the  addition  of  two  vertical  carbon  electrodes,  CC. 
The  furnace  is  heated  by  two  electric  arcs,  one  between  each  elec- 
trode and  the  slag  or  melted  metal  beneath  it.  The  current  passes 
down  one  electrode,  through  the  metal  and  up  the  other  electrode. 

The  lining  of  the  furnace  is  constructed  of  dolomite  bricks,  B,  and 
crushed  dolomite,  L.  A  is  the  roof,  made  of  silica-brick,  and  M  is  the 
molten  steel,  which  is  covered  with  a  layer  of  slag  S,  as  in  the  ordi- 
nary gas-fired  furnace.  The  furnace  is  built  in  a  steel  case  or  jacket, 
and,  unlike  the  open-hearth  furnace,  the  roof,  A ,  is  also  covered  with 
steel  plates,  E,  and  is  provided  with  eyes,  not  shown  in  the  figure,  by 
which  it  may  be  lifted  off  the  furnace.  The  weakest  part  of  the  roof 
is  around  each  electrode,  and  this  part  has  been  strengthened  by 
water-jackets,  /,  which  enable  a  closer  fit  to  be  maintained  round 
the  electrode,  and  so  reduce  the  loss  of  heat  and  prevent  the  exposed 
parts  of  the  electrodes  from  becoming  red  hot,  and  wasting  in  the  air. 
As  an  alternating  current  is  used,  it  is  not  desirable  to  have  iron  or 
steel  plates  on  the  part  of  the  roof  between  the  electrodes  CC,  as  this 
would  increase  the  inductance  of  the  electric  circuit  and  lower  the 
power-factor  of  the  furnace;  bronze  plates,  F,  are  therefore  used  to 
cover  this  part  of  the  roof.  The  charging  doors,  DD,  in  this  furnace 
are  placed  at  the  ends.  The  electrodes  are  square  in  cross-section, 
and  are  vertical  when  the  furnace  is  upright,  but  on  account  of  the 
tilting  motion  of  the  latter,  they  cannot  be  suspended  as  in  the  ore- 
European  Commission  Report,  1904,  Fig.  4. 


STEEL  FROM  METALLIC  INGREDIENTS 


215 


smelting  furnaces,  but  are  held  in  adjustable  holders,  Fig.  47,  which 
are  supported  by  the  furnace,  so  that  the  height  of  each  electrode  in 
the  furnace  is  unaffected  by  the  tilting  movement.  The  lower  end 
of  each  is  kept  a  short  distance  above  the  slag,  leaving  a  space  for  the 
electric  arc,  and  the  current  is  regulated  by  raising  or  lowering  the 
electrodes.  This  adjustment  is  effected  by  automatic  machinery 
controlled  by  the  voltage  of  the  furnace;  and  in  order  that  the  two  arcs 
may  be  kept  equal,  each  electrode  is  operated  separately,  being  con- 
trolled by  the  voltage  between  itself  and  the  metal  in  the  furnace. 


FIG.  93. — Heroult  steel  furnace. 


The  hearth  is  lined  with  dolomite  or  magnesite,  either  of  which  has 
the  advantage  of  being  more  refractory  than  silica-brick,  and  of 
allowing  strongly  basic  slags  to  be  used  for  removing  phosphorus  and 
sulphur  from  the  steel. 

The  Heroult  furnace  is  very  much  smaller  than  the  usual  open- 
hearth  furnace,  the  one  at  La  Praz  being  about  7  ft.  by  4  ft.,  inter- 
nally, and  taking  a  charge  of  only  3  tons,  while  the  furnace  at  Kort- 
fors  was  a  little  larger  than  this.  For  products  like  crucible  steel 
the  small  size  of  the  electric  furnace  may  be  no  disadvantage,  but 
when  it  is  desired  to  turn  out  structural  or  rail-steel,  larger  furnaces 
have  to  be  employed  to  compete  with  the  50- ton  open-hearth  furnace. 


216  THE  ELECTRIC  FURNACE 

The  Haanel  Commission  saw  the  process  of  making  both  low- 
and  high-carbon  steel  in  the  furnace  at  La  Praz,1  by  melting  mis- 
cellaneous steel  scrap,  purifying  it  by  repeated  additions  of  iron-ore 
and  lime,  and  then  making  suitable  additions  to  obtain  the  required 
percentage  of  carbon,  manganese  and  silicon.  The  scrap  contained 
0.055  Per  cent,  of  sulphur  and  0.22  per  cent,  of  phosphorus,  while 
the  final  steel  contained  only  0.02  per  cent,  of  sulphur  and  0.009 
per  cent,  of  phosphorus,  the  carbon  being  0.08  per  cent,  and  i.o  per 
cent,  in  the  two  steels.  The  scrap  was  melted  with  some  ore  and 
lime,  and  when  fusion  was  complete  the  slag  was  poured  off  and  a 
second  slag  was  made  by  adding  lime  with  a  little  sand  and  fluor-spar 
as  fluxes.  The  second  slag  was  poured  off  and  a  third  slag  made  in 
the  same  way  before  the  final  additions  were  made  to  the  steel.  The 
steel  is  more  completely  purified  in  this  way,  by  the  repeated  addi- 
tion of  fresh  slag-forming  materials,  than  if  the  whole  amount  were 
added  at  once.  In  making  the  low-carbon  steel  some  f  err o- manganese 
was  added  in  the  furnace  and  a  little  aluminium  in  the  ladle,  while 
in  making  the  high-carbon  steel  there  was  also  added  in  the  furnace 
some  "  carburite,"  which  is  a  mixture  of  iron  and  carbon,  and  some 
f  err  o- silicon.' 

The  electrical  power  employed2  was  about  350  kw.  in  each  opera- 
tion; the  voltage  was  no,  and  the  current,  which  was  not  measured, 
would  probably  be  about  4,000  amperes.  The  electrical  energy 
per  ton  of  steel  was  0.17  h.p.  years.3  The  time  required  was  5  hours 
for  a  small  charge  of  i. 25  tons  of  low-carbon  steel,  and  eight  hours 
for  2.33  tons  of  high-carbon  steel.  During  the  first  part  of  the 
operation,  before  the  steel  scrap  is  melted,  the  current  fluctuates 
violently  and  is  regulated  by  hand;  but  after  the  steel  has  melted 
around  the  electrodes  the  current  becomes  more  steady,  heating 
by  an  arc  beneath  each  electrode,  and  automatic  regulation  can  be 
employed.  The  full  power  was  not  applied  until  about  an  hour 
after  the  start.  Mr.  Harbord  states4  that  the  high-carbon  steel  is 
as  good  as  corresponding  grades  of  crucible  steel,  and  there  appears 
to  be  no  reason  why,  in  localities  where  water-power  is  cheap,  this 
furnace  should  not  replace  the  crucible  furnace  and  open-hearth 
furnace  for  the  manufacture  of  tool  steels  and  other  special  varieties 

1  European  Commission  Report,  pp.  70-72,  charges  658  and  660. 

2  European  Commission  Report,  pp.  53-55,  charges  658  and  660. 

3  Better  results  have  been  obtained  more  recently,  see  Table  XVI. 
The  long  ton  of  2,240  Ib.  is  employed  in  this  chapter. 

4  European  Report,  pp.  85-89,  and  p.  115. 


STEEL  FROM  METALLIC  INGREDIENTS  217 

of  steel  in  which  quality  rather  than   quantity  or  cheapness  is 
aimed  at. 

More  recent  data  with  regard  to  the  operation  of  the  Heroult 
steel  furnace  are  given  by  Professor  Eichhoff,  of  Charlottenburg.1 
He  furnishes  a  number  of  figures  for  the  output  and  power  con- 
sumption of  Heroult  furnaces,  from  which  the  following  may  be 
quoted: 

TABLE  XVI.— OPERATION  OF  5-TON  HEROULT  FURNACE 

Generator  Length    kw.-hr. 

capacity  of  heat    per  ton 

{Drawing       1      f  once 6.05  hr.       725 
slag.                \     \   twice 6.63hr.       795 
j      [  thrice..  .7.22  hr.       868 

{With  only  one  slag 2.o8hr.  219 
Drawing  \  /  once 2.57hr.  265 
slag.  /  \  twice....  3.15  hr.  324 

With  regard  to  the  possibility  of  making  structural  steel  in  the 
Heroult  furnace,  it  should  be  remembered  that  the  material  of  the 
charge  would  be  largely  pig-iron  and  ore,  as  there  would  not  be  suffi- 
cient scrap  available,  and  this  would  increase  the  time  and  electrical 
energy  required  for  the  operation.  On  the  other  hand  the  pig-iron 
could  be  charged  molten,  and  the  purification  of  the  metal  need  not 
be  carried  so  far  as  was  necessary  for  tool  steel,  while  the  larger 
scale  of  the  furnace  would  also  reduce  the  consumption  of  electrical 
energy  per  ton  of  product.  A  50- ton  furnace  might  be  expected  to 
require,  with  a  cold  charge,  about  5,000  kw.,  or  about  50,000  amperes 
at  no  volts;  while  40,000  amperes  might  be  sufficient  if  molten 
pig-iron  were  used. 

The  cost  of  making  structural  steel  in  a  5o-ton  Heroult  furnace, 
if  a  furnace  of  this  size  could  be  successfully  operated,  would  prob- 
ably, with  electrical  energy  at  $10  a  horse-power  year,  be  about  the 
same  as  in  a  gas-fired  open-hearth  furnace  using  coal  at  $3  a  ton. 
Assuming  that  the  general  cost  of  operating  the. two  furnaces  was 
the  same,  there  remains  for  the  Heroult  electric  furnace  the  cost 
of  electric  energy,  which,  at  o.io  horse-power  years  per  ton  would  be 
$i  per  ton,  and  the  cost  of  electrodes,  which  are  stated  to  cost  20 
cents  per  ton;  while  for  the  open-hearth  furnace  there  is  the  cost  of 
coal,  which  at  700  Ib.  per  ton  would  be  $i  and  the  cost  of  operating 

1  Stahl  und  Eisen,  1907,  No.  2,  pp.  50-58,  and  Dr.  Haanel's  1907  Report,  pp. 
139-146. 

2  Molten  steel  from  the  open-hearth  furnace. 


218  THE  ELECTRIC  FURNACE 

the  gas  producers  and  checker  chambers,  which  would  more  than 
balance  the  cost  of  electrodes.  Until  larger  furnaces  have  been  built, 
it  is  not  worth  while  to  attempt  to  estimate  in  detail  the  cost  of 
operating  them,  but  the  figures  given  are  enough  to  show  that  under 
favorable  conditions,  large  electric  furnaces  might  be  expected  to 
compete  with  gas-fired  furnaces  for  the  manufacture  of  structural 
steel. 

A  Heroult  furnace  for  the  production  of  50  tons  of  steel  a  day 
was  installed  in  the  plant  of  the  Halcomb  Steel  Co.,  in  Syracuse. 
The  furnace  is  used  in  conjunction  with  gas-fired  furnaces,  and  is 
charged  with  molten  superoxidized  steel  from  a  Wellman  furnace, 
the  operation  of  refining  being  completed  in  the  electric  furnace. 

An  illustrated  description  of  such  a  plant  appeared  in  the  Electro- 
chemical Industry,  vol.  v,  p.  272,  from  which  the  following  particu- 
lars are  taken:  The  steel  is  made  from  scrap,  etc.,  in  a  Wellman 
open-hearth  furnace  holding  25  tons.  The  operation  is  carried 
further  than  in  ordinary  open-hearth  practice,  until  the  carbon  and 
phosphorus  have  been  almost  entirely  eliminated;  the  removal  of 
the  sulphur  and  oxygen  and  the  recarburization  of  the  steel  being 
effected  in  the  Heroult  furnace.  Four  tons  of  highly  oxidized  metal 
from  the  open-hearth  furnace  are  transferred  to  the  electric  furnace, 
which  requires  one  and  one-half  hours  to  finish  this  charge  of  steel, 
and  has  a  daily  output  of  60  tons.  At  the  high  temperature  of  the 
electric  furnace  very  basic  slags  can  be  used  which  will  remove  very 
thoroughly  any  sulphur  remaining  in  the  steel,  and  in  the  neutral 
atmosphere  of  this  furnace  the  steel  can  be  deoxidized  far  more 
completely  than  is  possible  in  open-hearth  practice,  the  slag  on  the 
molten  steel  becoming  quite  neutral  or  free  from  iron  oxide.  Such 
steel  will  be  more  sound  than  the  usual  open-hearth  product.  The 
location  of  the  plant  is  not  specified,  and  no  figures  are  given  for 
the  amount  of  power  employed. 

The  uses  of  the  Heroult  steel  furnace  may  be  stated  as  follows: 

(a)  The  production  of  tool-steel  and  other  high-grade  steels  by 
melting  pure  materials  just  as  in  the  crucible  process.     Electric- 
furnace  steel  is  less  expensive  than  crucible-steel,  and  is  also  sounder 
and  more  tough. 

(b)  The  production  of  high-grade  steel  from  less  pure  materials 
by  keeping  them  in  a  molten  condition  beneath  oxidizing  slags  which 
are  repeatedly  changed  until  all  the  impurities  are  removed.     In 
this  process  the  pig-iron  which  forms  a  part  of  the  charge  will  pref- 
erably be  supplied  from  the  blast-furnace  in  the  molten  state. 


STEEL  FROM  METALLIC  INGREDIENTS  219 

(c)  The  electric  furnace  may  be  used  for  finishing  steel  which  has 
been  practically  freed  from  carbon  and  phosphorus  in  the  Bessemer  or 
open-hearth  furnace.1 

The  following  special  features  of  this  electric  furnace  may  be  no- 
ticed: 

(a)  The  high  temperature  of  the  furnace,  which  enables  very  basic 
slags  to  be  used. 

(b)  The  ability  to  exclude  the  air  and  to  finish  the  charge  under 
slags  which  are  practically  free  from  oxide  of  iron,  thus  obtaining  a 
sounder  product. 

(c)  The  slag  is  considerably  hotter  than  the  metal  and  will  there- 
fore be  fluid  enough  to  act  freely  on  the  metal  without  the  latter  being 
over-heated.     With  regard  to  the  possible  over-heating  of  the  steel  in 
electric  furnaces  nothing  definite  appears  to  be  known,  but  it  is  con- 
sidered that  if  steel  is  over-heated  in  the  presence  of  basic  slags  it  will 
absorb  nitrogen  and  become  less  tough  in  consequence.2    In  the  elec- 
tric furnace,  however,  even  nitrogen  is  largely  excluded  by  the  gases 
arising  from  the  operation,  as  no  air  or  other  gas  need  be  introduced 
from  without. 

(d)  In  the  final  or  recarburizing  stage  in  the  electric  furnace,  the 
conditions  are  so  strongly  reducing  and  the  temperature  is  so  high 
that  calcium  carbide  is  formed  in  the  slag.     There  is  therefore  practi- 
cally no  waste  of  the  ferro-manganese  or  other  metallic  additions, 
which  are  of  course  made  in  the  furnace  itself  and  not  in  the  ladle. 

(e)  The  cost  of  the  electric  process  is  decidedly  less  than  that  of 
the  crucible  process,  and  special  varieties  of  steel  can  be  made  com- 
mercially in  the  electric  furnace,  in  places  where  cheap  power  can  be 
obtained.     The  largest  furnace  which  has  been  operated  up  to  the 
present  holds  about  15  tons,  and  the  number  of  kilowatt-hours  per 
ton  of  steel  produced  in  such  a  furnace  varies  from  800  or  900,  when 
cold  stock  is  employed  and  is  purified  by  repeated  treatments  with 
fresh  slags,  to  about  200  when  it  is  merely  required  to  finish  a  charge 
of  steel  from  the  open-hearth  furnace. 

Mr.  Heroult3  has  proposed  an  electrically  heated  steel  mixer  of 
300  or  400  tons  capacity,  to  receive  the  steel  from  a  number  of  open- 

1  See  P.  L.  T.  Heroult,  U.  S.  Patent  807,026,  Electrochem.  Industry,  vol.  iv, 
p.  31,  for  converting  pig-iron  into  high-grade  steel  by  the  Bessemer  converter 
followed  by  the  electric  furnace. 

2  Jour.  Iron  and  Steel  Inst.,  1905,  No.  2,  p.  777,  and  1906,  No.  4,  p.  923. 

3  P.  L.  T.  Heroult,  U.  S.  patent  807,027,  see  Electrochem.  Industry,  vol.  iv, 
1906,  p.  30. 


220  THE  ELECTRIC  FURNACE 

hearth  or  Bessemer  furnaces,  thus  ensuring  a  uniform  product,  and 
allowing  a  more  perfect  deoxidation  of  the  steel  and  separation  from 
the  slag  than  by  the  usual  process  of  casting.  Prof.  Richards  has 
suggested  the  use  of  electrical  heating  as  an  auxiliary  in  an  ordinary 
open-hearth  furnace,  for  raising  the  temperature  of  the  steel  through 
the  last  100°  or  200°  C.  before  tapping,  as  a  little  electrical  heat  for 
reaching  the  highest  temperature  would  sometimes  save  a  good  deal 
of  time  and  fuel. 

Fifteen-ton  Heroult  Steel  Furnace. — An  important  recent  develop- 
ment of  the  Heroult  furnace  is  a  1 5-ton  furnace  which  has  been  in 
operation  at  South  Chicago  since  May  7,  iQoS.1 

This  furnace,  which  is  shown  diagrammatically  in  Fig.  94  and  in  side 
view  in  Fig.  95, 2  is  built  in  a  steel  shell  and  mounted  on  rockers  for 
pouring.  It  is  lined  with  magnesite  brick,  with  an  inner  lining  of  mag- 
nesite  mixed  with  one-fourth  of  its  weight  of  open-hearth  slag  and 
tamped  in  with  the  addition  of  tar.  The  furnace  is  provided  with 
three  carbon  electrodes,  entering  through  holes  in  the  roof,  and  is  oper- 
ated by  means  of  three-phase  current.  The  electrodes  are  held  in 
water-cooled  copper  castings  which  move  up  and  down,  each  being 
lifted  by  two  chains  passing  over  pulleys  and  actuated  by  an  electric 
motor  j  the  electrode-holders  are  guided  by  vertical  rods.  The  regu- 
lation of  the  electrodes  may  be  by  hand,  but  is  usually  automatic  by 
means  of  an  electrical  device  actuated  by  a  proportional  part  of  the 
current  passing  to  each  electrode.  The  roof  of  the  furnace  is  a  12- 
in.  arch  of  silica-brick.  The  electrodes  used  have  varied  from  24  in. 
in  diameter  to  about  1 1  in.  square.  The  electrical  power  is  supplied 
by  25-cycle  three-phase  generators,  at  2,200  volts.  It  is  slepped 
down  at  the  furnace  by  three  75o-kw.  transformers  giving  a  voltage 
of  80,  90,  100,  or  no  volts  by  means  of  taps  on  the  primary  wind- 
ings. Ninety  volts  are  usually  employed. 

This  furnace  is  generally  used  to  finish  steel  which  has  been 
blown  in  the  Bessemer  converter.  The  steel  is  blown  until  the  car- 
bon, silicon,  etc.,  are  practically  all  removed.  It  is  then  brought 
in  a  ladle,  the  slag  is  skimmed  off,  and  the  metal  is  poured  into 
the  electric  furnace.  Lime  and  iron-ore  are  added  to  make  a  basic 
slag  for  the  removal  of  the  phosphorus  which  in  the  original  metal 
is  about  o.i  per  cent.  This  slag  is  allowed  to  act  on  the  metal  for 
about  half  an  hour,  during  which  time  the  phosphorus  is  reduced 
to  about  o.oi  per  cent.  The  removal  of  the  phosphorus  is  effected 

1  C.  G.  Osborne,  Trans.  Am.  Electrochem.  Soc,,  vol.  xix,  1911,  p.  205. 

2  Reproduced  from  Mr.  Osborne's  paper. 


STEEL  FROM  METALLIC  INGREDIENTS 


221 


by  the  oxidizing  action  of  the  iron-ore,  and  the  lime  is  needed  to 
combine  with  the  oxidized  phosphorus  and  retain  it  in  the  slag. 
The  furnace  is  then  tilted  slightly  and  the  slag  is  raked  out,  thus 
getting  rid  of  the  phosphorus.  A  fresh  slag  is  made  by  adding  lime 
and  fluor-spar.  Powdered  coke  is  then  added  around  the  elec- 


FIG.  94. — 15-ton  Heroult  furnace. 

trodes  and  the  carbon  of  the  coke  combines  with  the  lime  in  the 
slag,  forming  calcium  carbide.  The  calcium  carbide  reacts  with 
the  sulphur  in  the  steel,  forming  calcium  sulphide  which  passes  into 
the  slag,  so  in  this  way  both  the  phosphorus  and  the  sulphur  are 
very  completely  eliminated.  This  second  stage  of  the  process  also 
serves  to  remove  from  the  steel  a  large  quantity  of  oxygen  or  oxide 
of  iron  which  was  dissolved"!]?  it,  and  which  could  not  be  allowed  to 


222 


THE  ELECTRIC  FURNACE 


remain  in  the  finished  metal.  After  the  sulphur  and  oxygen  have 
been  removed,  suitable  amounts  of  ferro-silicon,  ferro-manganese 
and  recarbonizer  are  added  for  the  purpose  of  introducing  into 
the  steel,  the  necessary  amount  of  manganese,  silicon,  and  carbon. 
The  furnace  is  then  tilted,  pouring  the  metal  into  a  ladle  from  which 
it  is  run  into  molds  through  an  opening  in  the  bottom  of  the  ladle. 


FIG.  95. — i5-ton  furnace  tilted. 

It  may  be  added  that  in  order  to  remove  the  phosphorus  the  carbon 
in  the  steel  must  be  very  low,  hence  the  need  of  recarbonizer  to 
impart  the  necessary  carbon  to  the  steel.  The  whole  time  taken 
by  this  operation  varies  from  one  and  one-half  to  two  hours  according 
to  the  grade  of  steel  required. 

The  power  supplied  to  the  furnace  may  be  as  much  as  2,000  kw., 
although  it  is  found  that  the  operation  of  keeping  molten  steel 
without  loss  of  heat  in  the  furnace  only  requires  about  750  kw. 
The  current  supplied  to  each  electrode  when  using  2,000  kw.  at 
90  volts,  assuming  a  power  factor  of  0.85,  would  be  15,000  amperes 


STEEL  FROM  METALLIC  INGREDIENTS  223 

per  electrode.  To  calculate  the  diameter  of  a  round  carbon  elec- 
trode for  this  current,  by  Hering's  method,  we  must  make  certain 
assumptions.  The  author  considers  that  it  will  be  satisfactory 
to  measure  the  electrode  from  the  holder  to  a  point  just  inside  the 
roof,  assuming  the  temperature  of  the  electrode  at  that  point  to 
be  that  of  the  furnace,  say  1,500°  C.  Another  method,  perhaps 
equally  correct,  is  to  measure  from  the  holder  to  the  lower  end  of 
the  electrode,  and  to  take  the  temperature  of  the  electrode  at  that 
point  as  that  of  the  electric  arc,  3,500°  C.  In  the  furnace  shown  in 
Fig.  94  the  lengths  are  3  ft.  to  a  point  just  inside  the  roof,  and  5  ft. 
to  the  lower  end  of  the  electrode,  and  the  corresponding  diameters 
of  carbon  electrodes  are  found  to  be  20.2  in.  and  20.3  in.1  In  addi- 
tion to  water-cooling  the  electrode-holders,  a  water-cooled  collar 
is  placed  on  the  roof  immediately  around  each  electrode!  This 
serves  to  cool  the  electrode  and  to  prevent  the  exposed  portion 
from  becoming  red  hot.  The  collars  also  assist  in  keeping  the  fur- 
nace roof  air-tight.  The  amount  of  electrical  energy  employed 
for  refining  i  ton  of  steel  is  not  stated,  but  must  be  in  the  order 
of  300  kw. -hours. 

This  furnace  has  been  in  operation  for  a  considerable  time  and 
has  been  used  to  make  all  kinds  of  steel,  both  high-  and  low-carbon, 
as  well  as  special  alloy  steels. 

The  electric  furnace  is  better  than  the  open-hearth  furnace  for  the 
production  of  steel  that  is  very  low  in  phosphorus  and  sulphur.  In 
addition  to  this  it  is  found  that  electric-furnace  steel  is  better  mechan- 
ically than  steel  of  similar  composition  made  in  the  open-hearth 
furnace.  This  superiority  appears  to  depend  on  the  greater  freedom 
from  oxides  and  the  greater  density  of  the  electric-furnace  steel. 

The  Keller  furnace,2  Fig.  96,  is  a  series-arc  furnace  having  four 
electrodes.  The  furnace  is  supported  on  a  train  of  rollers  and  can 
be  tilted,  like  the  Campbell  open-hearth  furnace,  by  means  of  an 
hydraulic'  ram.  The  electrode  holders  are  not  attached  to  the 
furnace,  but  are  suspended  from  independent  supports,  and  the 
furnace  cannot  be  tilted  until  the  electrodes  have  been  withdrawn 
The  system  for  supporting  the  electrodes  is  shown  in  the  figure. 

Each  electrode  hangs  from  the  end  of  a  hinged  arm,  A ,  and  two  of 

1  The  electrodes  are  actually  somewhat  larger  than  this,  24  inches  for  instance, 
because  large  electrodes  convey  the  heat  of  the  arc  to  the  metal  better  than  small 
ones,  protecting  the  roof  from  radiation,  and  are  less  subject  to  accident. 

2C.  A.  Keller,  "A  Contribution  to  the  Study  of  Electric  Furnaces  as  applied 
to  the  Manufacture  of  Iron  and  Steel,"  Trans.  Am.  Electrochem.  Soc.,  xv,  1909, 
p.  no. 


224 


THE  ELECTRIC  FURNACE 


FIG.  96. — Keller  steel  furnace. 


STEEL  FROM  METALLIC  INGREDIENTS  225 

these  arms  are  supported  on  each  of  the  four  pillars  PP.  There  are 
thus  eight  electrodes,  of  which  four  are  in  use  at  once  and  the  other 
four  are  ready  to  replace  these  as  they  become  too  short  for  further 
use.  To  replace  an  electrode  it  is  lifted  up  to  clear  the  furnace, 
swung  out  of  the  way  and  a  fresh  electrode  swung  over  the  furnace 
and  lowered  into  position.  Electrical  connection  with  the  electrode 
holder  is  made  by  the  flexible  metallic  conductors,  CC,  which  fold 
up  when  the  electrode  is  raised;  a  vertical  rod,  serving  to  keep  them 
in  place.  Electrical  connection  to  these  conductors  is  made  by  a 
fixed  system  of  bus-bars,  B,  with  which  each  electrode  arm  engages 
when  it  is  swung  into  position.  The  electrodes  are  therefore  auto- 
matically disconnected  from  the  electrical  supply  when  removed 
from  the  furnace  for  changing.  The  four-electrode  furnace  shown, 
having  two  pairs  of  series-arcs,  has  nearly  perfect  symmetry  of  heat- 
ing. It  has  also  the  advantage  that  the  electrical  supply  bus- 
bars, being  all  brought  to  the  same  point,  can  be  thoroughly  inter- 
laced and  thus  the  inductance  of  the  system  can  be  kept  low  and  a 
power  factor  can  be  as  high  as  0.97  with  12,000  amperes. 

In  the  four-electrode  Keller  furnace  simple  alternating  current  is 
used,  and  each  electrode  must  be  in  correct  adjustment  in  order  to 
maintain  the  right  voltage  in  the  furnace,  to  equalize  the  voltage  in 
the  arcs  which  are  in  series,  and  to  equalize  the  current  in  the  elec- 
trodes. This  is  accomplished  by  hydraulic  control  of  the  electrodes. 
They  can  be  lifted  or  lowered  individually  or  in  certain  combina- 
tions. It  would  probably  be  better  to  use  two-phase  current,  each 
phase  being  supplied  to  a  pair  of  electrodes  situated  at  opposite 
corners  of  a  square.  Mr.  Keller  suggests  the  use  of  three-phase 
current  using  three  or  six  electrodes. 

The  furnace  is  used  to  finish  steel  which  has  been  made  in  the  open- 
hearth  furnace.  A  charge  of  7.5  tons  of  steel  containing  0.15  per 
cent,  carbon,  0.06  per  cent,  sulphur,  and  0.007  Per  cent,  phosphorus, 
required  two  hours  45  minutes  in  the  furnace  with  an  average  power 
of  750  kw.,  yielding  a  steel  having  0.443  Per  cent,  carbon,  0.009  per 
cent,  sulphur  and  0.008  per  cent,  phosphorus.  The  energy  consumed 
per  ton  was  275  kw. -hours.  The  electrode  loss  was  about  26  Ib.  cost- 
ing 80  cents  per.  ton  of  steel.  This  figure  does  not  apparently  include 
the  cost  of  the  waste  ends  and  should  probably  be  somewhat  higher. 

SINGLE-ARC  OR  CONDUCTING-HEARTH  FURNACES 

Girod  Electric  Steel  Furnace. — Paul  Girod  started  electric  smelt- 
ing in  1898  with  a  plant  of  28  h.p.  making  ferro-alloys.  This 

15 


226  THE  ELECTRIC  FURNACE 

industry  grew  so  rapidly  that  by  1904  the  "Societe  Anonyme  Elec- 
trometallurgique,  Precedes  Paul  Girod"  had  three  large  plants  in 
Switzerland  and  Austria  making  ferro-alloys  (see  page  269).  Girod 
then  started  the  manufacture  of  steel  in  the  electric  furnace,  using 
at  first  a  resistance  furnace  in  which  he  produced  crucible  steel. 
Subsequently  he  designed  the  type  of  furnace  which  now  bears  his 
name.1  This  furnace,  Figs.  97  and  98,  is  an  arc-furnace  having  one 
or  more  depending  carbon  electrodes,  which  are  all  of  one  polarity; 
the  opposing  electrodes  being  a  number  of  vertical  steel  bars  passing 
through  the  bottom  of  the  furnace.  The  furnace  is  usually  square 
in  section  with  rounded  corners,  and  in  the  smallest  sizes  is  circular 
in  plan.  It  consists  of  an  iron  or  steel  casing  lined  with  refractory 
materials,  usually  magnesite  or  dolomite,  either  as  bricks  or  in  the 
form  of  a  rammed  lining.  The  cover  of  the  furnace  is  made  of  silica- 
bricks  and  can  be  lifted  off.  The  steel  rods  or  electrodes  in  the 
bottom  of  the  furnace  make  electrical  connection  with  the  molten  steel 
in  the  furnace.  They  are  surrounded  by  the  lining-material  so  that 
although  the  upper  few  inches  may  melt,  the  rods  remain  in  place. 
The  lower  ends  of  the  rods,  which  are  secured  to  the  iron  case  of  the 
furnace,  are  water-cooled  and  connected  to  the  electrical  supply.  The 
furnace  usually  rests  on  a  train  of  rollers  and  is  tilted  for  pouring  by 
means  of  a  lever  operated  by  a  hydraulic  ram.  The  furnace  has  one 
or  more  working  doors  and  a  spout.  A  small  furnace  of  about  2 
tons'  capacity  has  one  carbon  electrode  entering  through  the  roof.  In 
larger  furnaces  four  carbon  electrodes  are  used  which  are  all  con- 
nected in  parallel  to  the  same  pole  of  the  electric  supply.  The  hold- 
ers of  these  carbons  are  attached  to  standards  which  are  supported 
on  the  furnace  so  that  the  carbon  electrodes  tilt  with  the  furnace. 
Cast-iron  water-cooled  collars  are  placed  around  the  electrodes 
where  they  enter  the  roof  of  the  furnace  in  order  to  make  a  tighter 
joint  at  this  point  and  to  prevent  the  external  portions  of  the  elec- 
trodes becoming  red  hot  and  wasting  in  the  air. 

Comparing  this  furnace  with  a  series-arc  furnace  like  that  of 
Heroult,  the  Girod  furnace  is  simpler  in  operation,  particularly  in 
the  smaller  sizes  where  only  one  carbon  electrode  is  employed.  In 
starting  such  a  furnace,  there  is  only  one  electrode  to  be  regulated 
according  as  the  current  is  too  large  or  too  small,  while  in  the 
series-arc  furnace  there  are  two  electrodes  to  regulate  and  it  is 
essential  that  the  right  electrode  should  be  moved  when  regulation 
is  required.  In  starting  a  Girod  furnace  with  a  cold  charge  of  scrap- 
patented  1905.  See  Trans.  Am.  Electrochem.  Soc.,  vol.  xv,  p.  92. 


STEEL  FROM  METALLIC  INGREDIENTS  227 

iron,  the  electric  current  passes  through  the  whole  mass  of  material 
and  heats  this  by  small  arcs  at  all  the  poor  contacts  throughout  the 
mass. 

The  Girod  furnace  can  also  be  made  more  symmetrical  in  shape 
than  the  usual  series-arc  furnace,  although  the  same  symmetry  has 
been  obtained  by  Keller  in  a  series-arc  furnace  having  four  electrodes. 
The  Girod  furnace  has,  however,  the  complication  of  having  a  com- 
posite hearth. 

The  early  forms  of  Girod  furnace  had  a  voltage  of  50,  about  half 
that  of  a  series- arc  furnace,  and  this  meant  that  for  the  same  power 
twice  as  large  an  electric  current  must  be  employed,  thus  increasing 
the  losses  of  power  in  the  cables  and  connections  and  in  the  electrodes 
themselves.  In  some  larger  furnaces,  built  more  recently,  a  voltage 
of  75  has  been  obtained,  which  leaves  little  to  be  desired.  It  might 
appear  that  the  steel  rods  in  the  bottom  of  the  furnace  would  chill  the 
molten  steel,  especially  as  the  rods  are  water-cooled;  but  if  the  rods 
are  properly  proportioned  to  the  current  they  have  to  carry,  no  such 
chilling  action  will  take  place  as  long  as  the  current  is  flowing,  and 
under  these  conditions  less  energy  is  wasted  by  these  than  by  carbon 
electrodes.  Whenever  the  current  is  shut  off  for  any  reason,  there  will, 
of  course,  be  a  loss  of  heat  through  the  steel  rods.  Paul  Girod  speaks  of 
the  "thorough  electrification"  of  the  steel  as  being  one  advantage  of 
his  furnace.  This  means  that  the  electric  current  passes  right  down 
through1  the  steel  instead  of  merely  skimming  the  surface,  as  it  might 
be  expected  to  do,  in  a  series-arc  furnace.  It  is  not  likely  that  the 
passage  of  the  current  through  the  main  body  of  the  steel  would  pro- 
duce enough  heat  to  improve  materially  the  working  of  the  furnace, 
but  heat  will  be  produced  at  the  points  where  the  current  enters  the 
vertical  steel  rods,  as  in  the  Hering  furnace,  and  this  may  be  sufficient 
to  cause  a  circulation  of  the  steel  in  the  furnace.  A  serious  objection 
to  this  form  of  furnace  is  the  fact  that  the  electric  current  passes  com- 
pletely through  the  steel  ring  forming  the  walls  of  the  furnace  and 
this  increases  the  inductance  of  the  furnace  and  lowers  the  power- 
factor  in  a  way  that  does  not  occur  in  a  series-arc  furncae.  The 
difficulty  can  be  got  over  in  part  by  inserting  strips  of  copper  down 
the  sides  of  the  furnace,  but  this  complicates  the  construction  and  the 
operation  of  the  furnace. 

A  i2-ton  furnace  described  by  Borchers,1  and  shown  in  Figs.  97 
and  98,  is  about  12  ft.  square  and  5  ft.  high  outside.  Inside  it  is  10 

1  W.  Borchers,  "The  Girod  Furnace,"  Jour.  Iron  and  Steel  Inst.,  1910,  No. 
i,  p.  141. 


228 


THE  ELECTRIC  FURNACE 


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FIG.  97.— Girod  steel  furnace. 


STEEL  FROM  METALLIC  INGREDIENTS  229 

ft.  square  at  the  top,  5  ft.  6  in.  at  the  bottom  and  3  ft.  high.  The 
bottom  is  20  in.  thick,  the  sides  12  in.  and  the  roof  8  in.  It  has  four 
round  carbon  electrodes,  14  in.  in  diameter,  which  are  all  coupled  in 
parallel  to  the  same  pole  of  the  electrical  supply,  and  16  steel  rods 
which  are  coupled  in  parallel  to  the  other  pole  of  the  electrical  supply. 
These  rods  are  5  in.  in  diameter  at  their  upper  ends  and  7  in.  at  their 
lower  ends,  where  they  are  water-cooled.  They  originally  project 


FIG.  98. — Girod  steel  furnace. 

above  the  hearth  as  shown  in  one  figure,  but  melt  away  to  the  depth 
of  a  few  inches  when  the  furnace  is  in  operation.  The  furnace  uses 
1,000  to  1,200  kw.  at  70  to  75  volts.  This  corresponds  to  a  current  of 
about  17, oof>  amperes,  or  a  little  over  4,000  amperes  for  each  carbon 
electrode  and  1,000  amperes  for  each  steel  rod.  This  gives  a  current 
density  of  30  amperes  per  square  inch  in  the.  carbon  and  50  amperes 
per  square  inch  in  the  steel  rods.  A  calculation  shows  that  the  mini- 


230  THE  ELECTRIC  FURNACE 

mum  electrode  loss  would  be  obtained  with  carbon  rods  about  loin, 
in  diameter  and  steel  rods  about  i .  5  in.  in  diameter.  In  furnaces  of 
the  Girod  and  Heroult  types  it  is  usual  to  make  the  carbon  electrodes 
considerably  larger  than  is  required  by  theory,  in  order  to  make  pro- 
vision for  wasting  and  to  cover  the  arc  with  the  end  of  the  electrode, 
thus  protecting  the  roof  from  the  radiated  heat.  The  steel  rods  might 
apparently  be  made  smaller,  but  they  would  then  be  more  liable  to  be 
covered  up  during  the  patching  of  the  furnace. 

In  making  steel  from  cold  stock  in  this  furnace,  i  ton  of  steel 
requires  from  800  to  900  kw.-hours;  the  consumption  of  electrodes  is 
about  13  Ib.  per  ton;  the  lining  of  calcined  dolomite  will  last  for  80 
charges  before  any  repairs  are  needed.  The  bottom  will  last  from 
1 20  to  150  charges,  during  which  time  it  wears  down  about  4  in.  The 
coyer  of  the  furnace  lasts  for  about  25  charges.  The  cost  of  the  lining 
varies  from  80  cents  to  $i  per  ton  of  steel,  the  additions  such  as  lime, 
ore,  fluor-spar  and  ferro-alloys  cost  from  60  cents  to  $1.40  per  ton  of 
steel,  and  three  men  and  a  boy  are  required  to  operate  a  furnace  of 
this  size.  The  cost  of  electrodes  is  $i  per  ton  of  steel. 

The  following  Girod  furnaces  are  in  use  in  Europe:1 

At  the  Ugine  works,  France,  two  1 2-ton  and  three  2-ton  furnaces ; 
at  Rive-de-Gier,  France,  one  5-ton  furnace;  in  Switzerland,  one  2-ton 
furnace;  in  Belgium,  one  3-ton  furnace;  in  Germany,  one  2-ton 
furnace;  in  Austria,  one  3-ton,  one  1.5- ton  and  one  i/ 2-ton  furnace. 
W.  Leavitt  &  Co.,  New  York,  agents  for  this  furnace,  are  prepared  to 
guarantee  the  operation  of  a  25-ton  furnace. 

The  following  are  recent  papers  describing  the  Girod  furnace: 
"The  Girod  Electric  Furnace  for  the  Manufacture  of  Steel,"  by 
Paul  Girod.  Trans.  Amer.  Electrochem.  Soc.,  vol.  xv  (1909),  p.  127, 
and  Met.  and  Chem.  Eng.,  vol.  vii  (1909),  p.  259.  "The  Girod 
Furnace,"  by  W.  Borchers,  Jour.  Iron  and  Steel  Inst.,  No.  i,  1910, 
p.  141,  and  Met.  and  Chem.  Eng.,  vol.  viii  (1910),  p.  421. 

Keller  Steel  Furnace.2 — Charles  Albert  Keller,  who  is  the  manag- 
ing director  of  the  "Societe  des  Etablissements  Keller  Lele'ux," 
Livet  Works,  Isere,  France,  has  produced  a  furnace  similar  to  that  of 
Girod  but  having  a  composite  bottom  made  by  fixing  a  large  number 
of  slender  vertical  iron  rods  into  an  iron  plate,  and  ramming  lining 
material  between  these  rods.  This  bottom  appears  to  be  perfectly 
satisfactory  and  may  offer  certain  advantages  as  compared  with 

1  P.  Girod,  Electrochemical  Industry,  vol.  vii  (1909),  p.  259. 

2  C.  A.  Keller,  "A  Study  of  Electric  Furnaces,"  etc.,  Trans.  Am.  Electrochem. 
Soc.,  xv,  1909,  p.  97. 


STEEL  FROM  METALLIC  INGREDIENTS 


231 


that  of  the  Girod  furnace,  although  it  would  be  difficult  to  judge 
of  their  relative  merits  without  an  extended  acquaintance  with  the 
operation  of  both  of  these  furnaces.  A  Keller  furnace  having  this 
type  of  conducting  hearth  is  shown  in  Fig.  99.  The  hearth  is  made 
by  fastening  into  a  metal  base  plate  a  number  of  iron  rods  about 
i  in.  or  1.25  in.  in  diameter  and  about  2  ft.  long.  The  rods  are 
spaced  their  own  diameter  apart  and  a  refractory  material,  usually 
burnt  magnesite  agglomerated  with  tar 
or  pitch,  is  rammed  in  hot  between  the 
rods.  This  forms  a  very  strong  com- 
posite hearth  which  is  conducting  when 
cold  on  account  of  the  iron  rods,  and 
when  hot  the  upper  part  of  the  mag- 
nesite will  itself  become  a  conductor. 
The  remarks  made  with  regard  to  the 
Girod  hearth  apply  equally  to  the  Keller 
hearth.  A  conducting  hearth  will  tend 
to  give  more  uniform  heating  of  the 
charge,  but  in  general  will  be  worked  at  a 
lower  voltage  and  will  therefore  take  a 
larger  current  for  the  same  power,  thus 
entailing  larger  electrical  losses  in  the 
conductors.  The  electrode  holders  for 
this  furnace  are  supported  in  the  same 
manner  as  in  the  series-arc  Keller  fur- 
nace, Fig.  96.  The  furnace  can  be  oper- 
ated with  poly-phase  current,  using  the 
conducting  hearth  as  a  common  return. 

"Electro-Metals"  Steel  Furnace.— 
Messrs.  Gronwall,  Lindblad  and  Stal- 
hane  have  devised  a  steel-refining  fur- 
nace in  addition  to  their  ore-smelting  furnace  already  described. 

Their  refining  furnace,  Figs.  100  and  101,  resembles  in  general 
appearance  the  Heroult  furnace,  being  arranged  to  tilt  for  pouring 
and  being  provided  with  two  carbon  electrodes  entering  through 
the  roof  of  the  furnace  and  supported  on  adjustable  holders  attached 
to  the  shell  of  the  furnace. 

The  electrical  arrangements  are  different,  however.  Two-phase 
current  is  used  for  this  furnace  and  a  permanent  carbon  electrode  is 
built  into  the  bottom  of  the  furnace,  where  it  is  in  electrical  contact 
with  the  metallic  shell.  The  magnesite  lining  of  the  furnace  is  put 


FIG.  99. — Keller  steel  furnace. 


232 


THE  ELECTRIC  FURNACE 


FIG.  ioo. — "Electro-metals"  furnace. 


I  •  •  I  I  *  *  \s     WftMWiftft'""""*'"'1"1" 

W       W^  f 

FIG.  101. — "Electro-metals"  furnace. 


STEEL  FROM  METALLIC  INGREDIENTS  233 

in  over  this  carbon  block,  covering  it  completely.  The  two  upper 
electrodes  are  each  connected  to  one  phase  of  the  supply,  the  other 
pole  of  each  phase  being  connected  to  the  neutral  pole  or  bottom 
electrode  of  the  furnace.  In  starting  with  a  cold  furnace  the  current 
will  pass  between  the  two  upper  electrodes  through  the  metallic 
charge  but  when  the  furnace  becomes  thoroughly  heated  the  mag- 
nesite  begins  to  conduct  and  the  current  passes  in  its  normal  direc- 
tion, that  is  between  each  of  the  upper  electrodes  and  the  bottom 
electrode.  This  arrangement  is  more  favorable  to  steady  operation 
than  is  the  arrangement  of  the  Heroult  furnace  in  which  the  current 
enters  through  one  movable  electrode  and  leaves  through  the  other. 
In  the  Heroult  furnace  if  one  arc  is  broken  the  whole  supply  of  power 
is  stopped,  but  in  the  Gronwall  furnace  one  arc  may  be  broken 
without  interfering  with  the  other  and  therefore  the  furnace  will 
operate  more  steadily  as  regards  electrical  supply.  The  use  of  two- 
phase  current  produces  a  circulation  of  the  steel  in  the  furnace 
which  facilitates  the  refining  action  of  the  slag  lying  on  the  metal. 
It  is  stated  that  the  passage  of  the  current  through  the  magnesite 
bottom  of  the  furnace  does  not  impair  it  in  any  way.  Heat  will 
certainly  be  produced  by  the  passage  of  the  current  through  the 
magnesite,  and  will  assist  in  the  operation  of  the  furnace.  In  the 
event  of  such  a  furnace  being  supplied  with  three-phase  current 
this  is  transformed  into  low-voltage  two-phase  current  by  two 
transformers  arranged  with  the  Scott  connections  (Fig.  53)  as 
explained  in  the  case  of  the  Trollhattan  furnace.  The  rest  of  the 
construction  of  the  furnace  is  substantially  the  same  as  in  the  other 
varieties  of  Heroult  and  similar  furnaces  and  need  not  be  specially 
described. 

INDUCTION  FURNACES 

The  Kjellin  furnace  is  of  the  induction  type,  and  resembles  a 
step-down  transformer.  In  Fig.  io2l  which  represents  a  225-h.p. 
furnace  at  Gysinge,  Sweden,  A  is  the  primary  winding  to  which  an 
alternating  current  of  90  amperes  at  3,000  volts  is  supplied.  B 
is  a  circular  trough  containing  the  molten  steel,  and  corresponding 
electrically  to  a  secondary  winding  of  one  turn.  C  is  the  magnetic 
circuit  which  passes  through  both  the  primary  and  the  secondary 
windings.  The  alternating  current  in  the  primary  windings  induces 
an  alternating  current  in  the  ring  of  molten  steel;  this  secondary 

1  Dr.  Haanel,  European  Report,  Figs,  i  and  2.  and  pp.  1-4. 


234 


THE  ELECTRIC  FURNACE 


current  being  estimated  at  30,000  amperes  and  7  volts.  This 
furnace  has  the  great  advantage  of  requiring  no  electrodes,  which 
is  not  only  a  gain  as  regards  trouble  and  expense,  but  avoids  any  con- 
tamination of  the  steel  by  the  material  of  the  electrode.  The  heat 
is  generated  uniformly  throughout  the  steel,  which  is  contained  in  a 
closed  receptacle,  under  conditions  which  resemble  those  of  the 


0         I         2         3        4        5  FT., 


Section  N-N 
FIG.  102. — Kjellin  furnace. 

crucible  furnace.  The  electrical  furnace  has,  however,  the  advan- 
tage of  holding  as  much  steel  as  many  crucibles,  and  of  being  quite 
free  from  the  furnace  gases  which  are  liable  to  enter  even  a  closed 
crucible. 

Compared  with  the  Heroult  furnace,  the  Kjellin  furnace  has  the 
objection  that  the  annular  groove  containing  the  steel  is  very 
long  in  comparison  with  its  cross- section,  which  will  cause  the  loss 


STEEL  FROM  METALLIC  INGREDIENTS  235 

of  heat  to  be  excessive  and  the  weight  of  steel  to  be  small  for  a  fur- 
nace of  a  given  size.  The  furnace  does  not  form  a  very  efficient 
transformer,  and  it  appears  to  be  limited  in  size,  the  power-factor 
becoming  smaller  as  the  furnace  becomes  larger,  unless  the  frequency 
of  the  current  is  correspondingly  reduced.1  On  the  other  hand 
the  current  can  be  used  at  high  voltages,  such  as  3,000  or  even  5,000 
or  6,000  volts,  which  would  permit  of  the  generation  of  the  current 
and  its  transmission  over  moderate  distances  without  the  use  of 
a  step-down  transformer  at  the  furnace. 

The  Kjellin  furnace  was  in  operation  at  Gysinge,  Sweden,  when 
visited  by  the  Commission  in  1904,  and  was  usually  making  a  high 
class  of  tool  steel  from  pure  pig-iron,  steel  scrap  and  bar  iron,  for 
which  purpose  it  seems  particularly  adapted.  In  operating  the  fur- 
nace the  molten  steel  from  one  run  is  not  tapped  out  completely, 
but  about  one-third  of  it  is  left  in  the  groove  to  act  as  a  conductor 
to  carry  the  current  at  the  beginning  of  the  next  run;  the  fresh  charge 
of  charcoal  pig-iron  and  pure  iron  or  steel  scrap  is  added  to  the  super- 
heated steel  as  fast  as  it  can  take  it  without  chilling.  No  refining 
is  attempted  in  this  furnace,  the  operation  being  merely  one  of  melt- 
ing a  metallic  charge,  made  up  in  correct  proportions  to  give  a  steel 
of  the  right  composition. 

The  furnace  is  built  in  a  circular  iron  casing,  LL,  which  is  lined 
with  fire-brick  at  EE.  The  trough  B  is  surrounded  with  more  refrac- 
tory material,  DD,  for  which  either  magnesite  or  dolomite  bricks 
can  be  employed.  The  open  space  in  the  middle  of  the  brickwork 
serves  to  cool  the  primary  winding,  by  the  current  of  air  passing 
through  it.  Water-jackets  are  also  employed  to  protect  the  winding 
from  the  heat  of  the  furnace.  The  groove  B  is  covered  by  a  series 
of  movable  lids  to  retain  the  heat  as  far  as  possible,  and  any  of  these 
can  be  removed  for  charging  the  furnace.  At  the  end  of  the  opera- 
tion the  steel  is  tapped  from  the  furnace  by  the  spout  H. 

In  one  run2  the  furnace  contained  about  1,500  Ib.  of  steel  from 
the  previous  charge,  and  the  fresh  charge  of  best  Swedish  pig-iron, 
steel  scrap  and  Walloon  bar  iron  weighed  about  2,300  Ib.  Small 
amounts  of  silicon-pig  and  ferro-manganese  were  added  in  the 
furnace,  and  2,271  Ib.  of  good  quality  tool  steel  was  obtained. 
Samples  of  this  steel  were  tested  chemically  and  mechanically  by 
Mr.  Harbord  with  satisfactory  results.  The  power  employed  was 
nearly  150  kw.  and  the  run  lasted  6  hours.  The  energy  consumed 

1  In  more  recent  furnaces  this  difficulty  has  been  partly  overcome. 
2Dr.  Haanel,  European  Report,  Charge  No.  546,  pp.  59-61  and  47-48. 


236  THE  ELECTRIC  FURNACE 

amounted  to  0.13  h.p.  years,  or  850  kw.-hours  per  ton  of  steel  ingots. 
The  power- factor  at  full  load  was  only  0.635  with  a  current  frequency 
of  13  1/2  cycles  per  second.1  It  will  be  seen  that  the  consumption 
of  energy  for  a  ton  of  tool  steel  is  less  than  in  the  Heroult  furnace; 
but  it  must  be  remembered  that  in  the  latter,  miscellaneous  scrap 
was  employed  and  washed  with  basic  slags  until  free  from  phosphorus 
and  sulphur,  after  which  it  had  to  be  recarburized  to  obtain  tool 
steel,  while  in  the  Kjellin  furnace  only  the  purest  materials  were 
employed,  and  they  merely  needed  to  be  melted  together  in  order 
to  produce  steel.  On  account  of  its  smaller  capacity,  the  Kjellin 
furnace  will,  no  doubt,  use  more  electrical  energy  than  the  Heroult 
for  the  same  amount  of  useful  work,  but  this  difference  in  efficiency 
does  not  appear  to  be  very  great  and  may  be  more  than  offset  by 
the  absence  of  electrodes  with  their  regulating  appliances,  heavy 
cables,  and  low-voltage  transformers.  In  other  words,  the  Kjellin 
furnace  may  be  expected  to  hold  its  own  for  certain  classes  of  work, 
in  competition  with  the  Heroult  furnace. 

The  Kjellin  furnace  has  been  used  for  high-carbon  steel-making, 
but  attempts  were  made,  for  the  Commission,  to  make  medium- 
and  low-carbon  steel  in  this  furnace;  and  while  the  attempts  were 
not  very  successful,  mainly  because  there  was  not  sufficient  elec- 
trical power  to  melt  the  more  refractory  mild  steel,  it  appeared 
probable  that  with  a  little  more  power  any  variety  of  steel  could  be 
produced  in  the  Kjellin  furnace. 

During  the  year  ending  May  31,  1906,  a  furnace  at  Gy singe, 
giving  i  ton  of  steel  per  tap,  produced  950  tons  of  steel  and  special 
steel  ingots.2 

More  recent  data  with  regard  to  the  furnace  at  Gysinge  are  given 
by  the  American  Electric-furnace  Co.3  They  state  that  the  furnace 
requires  for  its  operation  165  to  170  kw.,  and  has  a  capacity  of  3,000 
Ib.  of  metal,  of  which  about  1,850  Ib.  are  tapped  out  at  the  end  of 
each  heat.  The  length  of  a  heat  is  four  hours,  and  the  consumption 
of  energy,  when  all  the  charge  is  added  cold,  is  800  kw.-hours  per 
ton.  Working  with  hot  metal  from  the  blast-furnace  a  larger  out- 
put and  greater  economy  is  obtained.  This  is  partly  because  the 

1  E.  C.  Ibbotson,  Jour.  Iron  and  "Steel  Inst.,  1906,  No.  Ill,  p.  397.    Electro- 
chemical Industry,  vol.  iv,  p.  350. 

2  Further  particulars  of  the  furnaces  at  Gysinge  are  contained  in  a  report  by 
V.  Engelhardt,  Stahl  u.  Eisen,  1905,  and  Electrochemical  Industry,  vol.  iii  (1905), 
p.  294. 

3  American  Electric  Furnace  Co.,  45  Wall  Street,  New  York.     Bulletin  No.  i, 
June,  1907. 


STEEL  FROM  METALLIC  INGREDIENTS  237 

furnace  can  be  completely  emptied  after  each  heat,  as  it  can  easily 
be  restarted  by  pouring  in  a  charge  of  molten  pig-iron.  Thus  a 
charge  of  1,430  Ib.  of  molten  pig-iron  was  poured  into  the  empty 
furnace,  and  2,860  Ib.  of  cold  pig  and  scrap  were  added.  In  six 
and  three-quarter  hours  with  182  kw.  the  charge  was  finished,  the 
consumption  of  energy  being  650  kw. -hours  per  ton  of  steel  ingots. 
The  waste  of  material  during  the  melting  operation  is  found  to  be 
2  per  cent.,  and  the  furnace  lining  will  last  for  twelve  weeks,  cost- 
ing about  60  cents  per  ton  of  steel.  Purchasing  electrical  energy 
at  1/2  cent  per  kilowatt-hour,  and  using  cold  material  in  the  furnace, 
the  electrical  energy  will  cost  a  little  more  than  the  fuel  in  the  cruci- 
ble process,  but  a  great  economy  is  effected  by  avoiding  the  use  of 
crucibles.  The  cost  for  labor  is  also  much  less  in  the  electrical 
method. 

The .  Colby  Steel  Furnace. — More  than  ten  years  before  the 
invention  of  the  Kjellin  furnace,  Mr.  Edward  Allen  Colby  had 
patented  an  induction  furnace  for  melting  metals.1  In  one  of  his 
first  patents,2  the  primary  winding  is  shown  surrounding  the  circular 
channel,  instead  of  being  within  it  as  in  the  Kjellin  furnace;  the 
furnace  tilts  in  order  to  pour  the  charge,  and  is  covered  with  a  hood 
for  the  purpose  of  excluding  the  air;  the  hood  being  arranged  so  that 
the  molten  metal  could  be  poured  into  a  mould  without  being  exposed 
to  the  air. 

In  his  later  furnaces,  however,  the  primary  winding  has  been  placed 
within  the  secondary  as  in  the  Kjellin  furnace,  and  the  covering  hood 
has  been  discarded,  but  the  arrangement  for  tilting  the  furnace  in 
order  to  pour  its  contents  is  still  employed. 

About  eight  years  ago  Mr.  Colby  and  Dr.  Leonard  Waldo3  pro- 
duced the  first  steel  made  in  the  induction  furnace  in  the  United 
States,  and  in  1907  a  Colby  furnace  holding  190  Ib.  of  crucible  steel 
was  in  operation  at  the  works  of  Henry  Disston  &  Sons,  near  Phila- 
delphia,4 and  several  much  larger  furnaces  were  in  process  of 
construction. 

The  small  furnace  used  at  the  Disston  steel  plant  is  shown  in 

1  U.  S.  patents  428,378,  428,379,  and  428,552,  of  May  20,  1890.     See  Electro- 
chemical Industry,  vol.  iii,  p.  134. 

2  U.  S.  patent  428,552,  see  Electrochemical  Industry,  vol.  iii  (1905);  Fig.  3, 
p.  299. 

3  Electrochemical  Industry,  vol.  iii  (1905),  p.  185. 

4  Trans.  Amer.  Electrochem.  Soc.,  vol.  xi  (1907). 
Electrochemical  Industry,  vol.  v(i907),  p.  232. 


238 


THE  ELECTRIC  FURNACE 


the  frontispiece,  and  diagrammatically  in  Fig.  103. 1  It  consists  of  a 
laminated  iron  core,  around  which  is  a  primary  winding  of  28  turns 
of  thick- walled  copper  tube,  P,  and  an  annular  crucible,  C,  containing 
the  steel,  S,  which  forms  the  secondary  circuit  of  the  transformer. 
The  whole  furnace  tilts  to  pour  the  molten  steel,  rotating  about  an 
axis  indicated  by  the  line  XY.  The  primary  winding  can  be  cooled 
very  efficiently  by  water  circulating  through  the  copper  tube,  of 
which  it  is  composed,  and  can  therefore  be  placed  in  close  proximity 
to  the  secondary  circuit  without  any  danger  of  becoming  over- heated. 
This  arrangement  of  the  coils  gives  far  less  opportunity  for  magnetic 
leakage  than  in  the  Kjellin  furnace  shown  in  Fig.  102,  and  it  is  not 
surprising  to  find  that  a  much  higher  power- factor  has  been  obtained; 


x 


Iron  Core 


Scale  of  Inches 

FIG.  103. — Colby  furnace.  &  „ 

although  this  may  be  due  in  part  to  the  small  size  of  the  Colby  fur- 
nace. Mr.  Colby  gives  the  power-factor  as  0.93,  and  states  that  the 
average  power-factor  after  the  charge  of  metal  is  fused  is  never 
below  o.  90.  The  use  of  copper  tube  for  the  primary  involves,  how- 
ever, the  employment  of  relatively  low- voltage  current,  and  its  prox- 
imity to  the  crucible  must  cause  considerable  losses  of  heat,  although 
this  will  be  guarded  against  as  far  as  possible  by  a  packing  of  asbestos 
or  other  heat-insulating  substance  between  the  crucible  and  the  cop- 
per pipes.  The  crucible  itself  is  made  of  graphite  and  clay,  being 
similar  in  composition  to  the  graphite  crucibles  usually  employed  for 
steel-making.2  Such  crucibles  are  moderately  good  conductors  of 

1  From  a  sketch  by  Mr.  Colby. 

2  For  construction  of  the  Colby  crucible  see  U.  S.  patents  840,825  and  840,826, 
described  in  Electrochem.  Industry  vol.  v,  p.  55. 


STEEL  FROM  METALLIC  INGREDIENTS  239 

%  * 

electricity,  and  a  portion  of  the  secondary  current  will  no  doubt 
pass  through  the  crucible  itself,  thus  producing  heat  in  the  walls 
of  the  crucible  as  well  as  in  the  steel..  The  crucible  rests  upon  a  slab 
of  soapstone,  N,  and  is  jacketed  by  some  heat-insulating  material,  /. 
The  following  data  in  regard  to  this  furnace  are  taken  in  part 
from  the  account  issued  to  the  American  Electrochemical  Society,1 
and  in  part  from  a  private  communication  from  Mr.  Colby  to  the 
author: 

Crucible  capacity,  200  Ib.  of  steel. 

Working  capacity,  100  Ib.  of  cast-steel  ingots  per  hour. 

Kilowatt  hours  per  100  Ib.  of  cast-steel  ingots,  35. 

Power-factor,  0.93. 

Maximum  kilowatts  with  190  Ib.  steel  seldom  exceeds  43. 

Rated  size  of  furnace  for  crucible  steel-making,  60  kw. 

Primary  current  is  single-phase,   24o-volt,  frequency  60,    less  than   200 

amperes. 

Secondary  current  about  9  volts  and  about  5,000  amperes. 
Length  of  operation,  i  hour;  half  of  which  is  required  for  fusion  and  the  other 

half  for  refining  and  "killing." 
Ingots  of  about  90  Ib.  are  poured  every  hour,  the  remainder  of  the  steel  being 

left  in  the  crucible  for  starting  the  next  operation. 
Primary  winding,  28  turns  of  copper  tube  of  3/8-in.  internal,  and  5/8-in. 

external  diameter. 

The  induction  furnace  is  an  extremely  convenient  and  reasonably 
economical  appliance  for  melting  steel  and  other  metals  and  alloys, 
and  there  can  be  no  doubt  that  when  mere  melting  is  required,  as  in 
the  production  of  crucible-steel  from  pure  varieties  of  iron  and  steel, 
it  is  the  best  form  of  electric  furnace;  and  that  when  electric  power  can 
can  be  obtained  at  reasonable  rates,  it  is  both  better  and  cheaper 
in  operation  than  the  crucible  process. 

The  larger  sizes  of  induction  furnace,  such  as  would  be  used  in 
the  production  of  structural  steel,  appear  to  have  a  reasonably 
good  efficiency.  A  furnace  of  636  kw.  is  stated  to  have  an  output 
of  30  tons  per  day  if  charged  with  cold  material,  and  36  tons  when 
charged  with  hot  metal.  These  figures  refer  to  the  production  of 
steel  from  "pig  and  scrap,"  that  is  by  a  simple  melting  operation, 
and  correspond  to  expenditures  of  590  and  490  kw.-hours  respectively 
per  ton  of  steel.  It  should  be  noted  that  these  figures  are  apparently 
the  results  of  calculations  by  Mr.  Engelhardt,2  and  not  of  actual 

1  Trans.  Amer.  Electrochem.  Soc.,  vol.  xi  (1907),  and  Electrochemical  Industry, 
vol.  v  (1907),  p.  232. 

2  Electrochemical  Industry,  vol.  iii,  p.  295. 


240  THE  ELECTRIC  FURNACE 

operations.  About  the  same  amount  of  electrical  energy  would 
probably  be  needed  for  the  simple  melting  of  pig  and  scrap  in  the 
Heroult  furnace,  but  as  this  furnace  is  generally  employed  for  puri- 
fying as  well  as  merely  melting  the  steel,  it  is  not  easy  to  make  an 
exact  comparison. 

The  Gronwall  Furnace. — An  induction  steel  furnace  invented 
by  Messrs.  Gronwall,  Lindblad  and  Stalhane,  is  illustrated  in  Fig. 
I04.1  This  furnace  embodies  certain  features  which  enable  it  to 
be  built  on  a  larger  scale  than  was  previously  possible,  without 
having  an  excessively  low  power-factor,  and  without  requiring 
current  at  unusually  low  frequencies. 

The  first  point  to  notice  is  the  trough  containing  the  steel.  In- 
stead of  being  circular  as  in  the  Kjellin  and  Colby  furnaces,  this  trough 
has  a  semicircular  portion,  F,  passing  around  the  core,  E,  and  a 
folded  portion,  G,  extending  to  the  right  of  the  core.  This  form  of 
channel  has  the  advantage  of  being  more  compact  than  a  circular 
channel  of  the  same  length,  and  of  having  a  smaller  inductance. 
The  gridiron-like  construction  of  the  channel  had  been  employed 
previously  by  G.  Gin,  Fig.  108,  but  its  application  to  an  induction 
furnace  was  new. 

Another  new  feature  in  the  furnace  is  the  position  of  the  primary 
coil,  which  is  placed,  not  at  C,  as  in  the  earlier  forms,  but  at  A, 
around  the  outer  limb,  D,  of  the  transformer  core.  B  and  C  are 
compensator  coils  for  reducing  the  magnetic  leakage  of  the  core. 

In  the  earlier  forms  of  induction  furnace  a  serious  difficulty  was 
the  very  low  power-factor,  which  appeared  to  limit  the  utility  of 
this  type  of  furnace.  Lindblad  gives  the  following  formula  for  the 
power-factor  of  an  induction  furnace; 


cna 

TanF~lT 


where, 


F  =  angle  of  phase  displacement. 
n=  frequency. 
.a  =  area  of  cross-section  of  steel  in  the  channel. 
/=  length  of  channel, 
s  =  specific  resistance  of  the  steel. 
c  =  a  constant. 

Ws  =  magnetic  resistance  around  secondary  circuit. 
Wp=  magnetic  resistance  around  primary  circuit. 

1  Dr.  Haanel's  1907  Report,  pp.  191-104,  and  plates  x,-xii. 


STEEL  FROM  METALLIC  INGREDIENTS 


241 


The  power-factor,  cos  F,  is  highest  when  tan  F  is  lowest,  that  is 
when  the  magnetic  resistances  are  high,  the  frequency  low,  and  the 
electrical  resistance  of  the  secondary  is  high.  The  very  low  power 
factors  of  the  earlier  furnaces  were  caused  by  the  excessively  low 
electrical  resistance  of  the  secondary  circuit,  and  the  necessarily 
large  space  within  the  circular  steel  channel,  which  afforded  an  easy 
leakage  for  the  lines  of  magnetic  force.  It  became  necessary  there- 
fore to  use  currents  of  very  low  frequency  such  as  12  or  15  for  small 
furnaces,  while  even  three  or  five  alternations  were  proposed  for 


tf-y— w 

FIG.  104. — Gronwall  furnace. 


larger  furnaces;  thus  requiring  special  electrical  machinery,  and 
making  it  impossible  to  draw  the  current  from  ordinary  power- 
plants. 

In  this  furnace  the  steel  channel  turns  as  closely  as  possible 
around  the  transformer  core,  and  yet  has  a  considerably  greater 
length,  thus  obtaining  an  increased  electrical  resistance  of  the 
secondary  circuit  and  a  greater  resistance  to  the  magnetic  leakage 
through  that  circuit.  The  primary  coil,  A,  is  placed  on  the  outer 
limb  of  the  core,  in  order  to  have  it  further  from  the  hot  metal 

16 


242  THE  ELECTRIC  FURNACE 

and  so  to  protect  the  insulation  of  the  coil  from  the  heat  of  the  fur- 
nace. This  arrangement  allows  of  the  use  of  higher  voltage  current 
in  the  primary  than  would  be  possible  in  the  old  position. 

The  compensator  coils,  B  and  C,  are  two  equal  coils  connected 
together  in  such  a  way,  that  if  the  magnetic  flux  in  D  and  E  were 
equal,  no  current  would  flow  in  B  or  C.  If,  however,  leakage  occurs, 
and  there  is  a  greater  flux  in  D  than  in  E,  a  current  will  flow  in  the 
coils  in  such  a  way  as  to  oppose  and  partly  prevent  the  leakage. 
An  external  source  of  electromotive-force  may  also  be  used  to  main- 
tain a  current  in  the  coil,  C,  which  then  becomes  an  auxiliary  of 
the  main  primary  coil,  A. 

The  electrical  deficiencies  of  the  transformer  or  induction  furnace 
may  be  made  clearer  as  follows:  In  an  ordinary  transformer  the 
electric  current  flowing  in  the  primary  coil  sets  up  a  magnetic  force 
in  the  core,  and  this  force  passes  through  both  the  primary  and  the 
secondary  circuits.  The  alternating  current  in  the  primary  is  con- 
stantly changing  in  amount,  and  corresponding  changes  take  place 
in  the  magnetic  force  in  the  core.  The  changes  in  the  magnetic 
force  produce  an  electric  current  around  the  secondary  circuit. 
In  the  ordinary  transformer  the  primary  and  secondary  windings 
are  close  together  and  the  magnetic  force  set  up  by  the  primary 
must  pass  through  the  secondary  also,  but  in  the  induction  furnace 
the  secondary  winding  is  a  ring  of  molten  steel  and  cannot  be  placed 
close  to  the  primary  winding  without  destroying  it.  The  magnetic 
force  produced  by  the  primary  has  therefore  a  chance  of  escaping 
its  work  by  doubling  back  between  the  primary  and  the  secondary 
coils.  The  arrangement  shown  in  the  figure,  of  one  coil  on  each 
limb  of  the  core,  makes  it  more  difficult  for  the  magnetic  force  to 
escape  without  doing  its  work  and  driving  an  electric  current  around 
the  ring  of  molten  steel.  Another  deficiency  of  the  induction 
furnace  arises  from  the  low  electrical  resistance  of  the  secondary 
circuit.  In  an  ordinary  transformer  the  secondary  winding  is  con- 
nected to  some  external  resistance  or  other  load,  but  in  the  furnace 
the  secondary  winding  is  short-circuited,  and  having  a  very  low  resist- 
ance, its  self-inductance  will  be  high  as  compared  with  its  ohmic 
resistance;  and  the  current  produced  in  the  steel  will  consequently 
be  far  less  than  it  would  be  if  the  secondary  circuit  were  non-induc- 
tive, or  the  heat  produced  will  be  less  than  it  would  be  if  the  ohmic 
resistance  of  the  secondary  formed  a  larger  proportion  of  the  whole 
resistance  of  that  circuit.  The  gridiron  portion  of  the  steel  channel 
has  a  smaller  inductance  in  proportion  to  its  length  than  the  circular 


STEEL  FROM  METALLIC  INGREDIENTS 


243 


part  of  the  channel  and  consequently  increases  the  non-inductive 
part  of  the  resistance  and  hence  the  effectiveness  of  the  transformer. 

An  alternative  device  for  preventing  magnetic  leakage  consists 
of  a  copper  shield  or  mantle  around  the  core,  as  shown  at  M  in  Fig. 
105,  where  A  is  the  primary  coil  and  DEH  the  iron  core.  The  lines 
of  magnetic  force  cannot  pass  easily  through  this  shield,  as  in  so  doing 
they  would  produce  eddy  currents  in  the  metal  of  the  shield,  and 
these  eddy  currents  would  oppose  the  magnetic  forces  which  started 
them.  The  shield  must  not,  however,  form  a  complete  ring  around 
the  core,  as  it  would  then  act  as  a  choking  coil  on  the  primary  cur- 
rent. It  must  therefore  be  made  in  the  form  of  an  incomplete 
cylinder,  as  at  (i),  or  a  spiral,  as  at  (2). 

The  furnace  in  Fig.  104  is  constructed  of  brickwork,  TV,  in  a  metal 
container,  and  the  groove  containing  the  molten  steel  is  constructed  in 


FIG.  105. — Induction  furnace  with  shielded  core. 

some  more  refractory  material,  M ,  such  as  magnesite.  The  furnace 
is  supported  on  two  piers,  PP,  between  which  space  is  left  for  the 
transformer  core  D  E.  Three  spouts,  S  S  S,  are  provided  at  different 
levels;  the  upper  spout  serving  to  remove  the  slag,  the  middle  spout 
to  tap  off  the  usual  amount  of  steel,  leaving  a  quantity  in  the  furnace 
for  keeping  the  secondary  circuit  unbroken,  and  the  lower  spout  for 
emptying  the  furnace  when  necessary.  In  operating  the  Gronwall 
furnace  partial  short-circuits  were  produced  by  the  leakage  of  steel 
into  the  lining  materal  dividing  adjacent  parts  of  the  folded  channel. 
A  simpler  design  of  secondary  channel  has  therefore  been  adopted. 

Ro'chling-Rodenhauser  Induction  Furnace. — The  use  of  the  in- 
duction furnace,  as  exemplified  in  the  Kjellin  and  Colby  furnaces,  is 
limited  by  the  very  low  power-factor  which  is  obtained  when  these 
furnaces  are  built  of  any  considerable  size,  and  by  the  necessity  of 
using  currents  of  very  low  frequency.  The  use  of  these  furnaces  is 


244 


THE  ELECTRIC  FURNACE 


also  limited  because  the  molten  steel  contained  in  a  narrow  channel 
cannot  be  subjected  to  refining  processes  involving  the  addition  of 
basic  slags.  The  Rodenhauser  furnace  overcomes  these  difficulties  in 


FIG.  106. — Rochling-Rodenhauser  furnace. 

a  large  measure.  The  single-phase  furnace  shown  in  Figs.  106  and  107 
has  an  iron  core  with  both  vertical  limbs,  H  H,  passing  through  the 
furnace;  each  of  these  limbs  is  surrounded  by  a  part  of  the  primary 
winding,  A,  and  around  this  is  the  channel,  C,  containing  the  molten 


FROM  METALLIC  INGREDIENTS  245 

steel.  These  channels  meet  in  the  middle  of  the  furnace,  at  D,  and 
are  expanded  at  this  point  to  form  a  chamber  of  considerable  dimen- 
sions where  a  large  quantity  of  steel  can  be  held  in  a  molten  condition. 
The  electric  current  circulating  around  each  limb  of  the  core  will 
produce  most  of  the  heat  in  the  narrow  channel,  C,  and  it  is  therefore 
necessary  to  provide  additional  means  for  heating  the  steel  in  the 
enlarged  central  chamber,  D.  This  is  done  by  means  of  "pole- 
pieces,"  E,  which  are  of  iron,  imbedded  in  the  walls  of  the  furnace, 
and  separated  from  the  molten  steel  by  a  portion,  G,  of  the  lining. 
The  lining  at  this  point  is  composed  largely  of  magnesite,  and  is 


FIG.  107. — Rochling-Rodenhauser  furnace. 

sufficiently  conducting  to  carry  the  electric  current  from  the  pole- 
piece  to  the  molten  steel. 

In  starting  the  furnace  heat  is  produced  in  the  annular  channels  as 
in  other  induction  furnaces,  but  when  the  furnace  has  become  heated 
the  current  also  passes  through  the  molten  steel  from  the  "  pole-pieces ' ' 
and  thus  sufficient  heat  is  supplied  to  the  central  chamber  to  keep  the 
steel  molten.  The  "pole-pieces"  are  connected  with  a  few  turns, 
B,  of  a  very  heavy  conductor  passing  around  the  limbs  of  the  core. 
It  seems  probable,  in  view  of  the  very  low  electrical  resistance  of  the 
body  of  steel  in  the  central  chamber,  that  a  material  portion  of  the 
heat  is  developed  by  the  passage  of  the  current  through  the  mag- 
nesite covering  of  the  "pole  pieces,"  but  in  any  case  the  heat  is  com- 
municated to  the  steel  and  serves  to  keep  it  molten. 


246  THE  ELECTRIC  FURNACE 

It  is  found  that  this  furnace  has  a  far  better  power-factor  than  a 
similar  furnace  of  the  Kjellin  type.  Rodenhauser  furnaces  have  been 
made  to  hold  8  tons  of  steel. 

A  furnace  for  three-phase  current  has  also  been  constructed.  In 
this  furnace  the  iron  core  has  three  vertical  limbs,  which  are  connected 
together  above  and  below  the  furnace.  Each  limb  has  a  primary 
winding  supplied  with  single-phase  current  and  the  channels  contain- 
ing the  steel,  which  lie  around  each  of  these  cores,  coalesce  in  the 
middle  of  the  furnace  to  form  a  central  chamber.  Iron  "pole- 
pieces"  are  provided  in  this  furnace,  as  in  the  single-phase  furnace, 
for  heating  the  steel  in  this  central  portion.  Both  single-phase 
and  three-phase  furnaces  are  arranged  to  tilt  for  pouring  the  steel  and 
the  slag.  The  use  of  three-phase  current  has  the  advantage  of  caus- 
ing a  circulation  of  the  steel  around  the  channels  in  the  furnace  and 
thus  insuring  a  homogeneous  product.  This  circulation  is  caused  by 
the  rotary  magnetic  field  of  such  a  furnace.  The  Rodenhauser  fur- 
nace, by  overcoming  the  difficulty  of  low  power-factor  and  by  pro- 
viding space  for  refining  the  metal  in  the  furnace,  has  enabled  the 
induction  furnace  to  be  used  for  all  purposes  for  which  the  arc-furnace 
is  suitable. 

Dr.  K.  G.  Frank  of  the  Siemens  &  Halske  Co.,  informs  the  author 
that  their  furnaces  are  now  operated  by  two-phase  instead  of  three- 
phase  current.  The  two-phase  furnace  has  a  "figure-eight"  hearth, 
like  the  single-phase  furnace,  but  has  two  magnetic  circuits. 

Frick  Induction  Furnace.1 — This  resembles  the  Kjellin  furnace, 
but  the  primary  windings  consist  of  flat  coils  lying  above  and  below 
the  furnace.  Furnaces  are  now  designed,2  of  the  double-ring  type 
but  without  the  pole  pieces.  Instead  of  these  Mr.  Frick  uses  his 
flat  coils  both  above  and  below  the  furnace,  and  also  a  Kjellin 
coil  wound  on  the  core;  the  furnace  being  operated  with  two-phase 
current. 

RESISTANCE  FURNACES 

The  Gin  Steel  Furnace.3 — Mr.  G.  Gin  invented  in  1897, 4  a  furnace 
in  which  the  heat  is  generated  by  the  passage  of  a  large  electric 

1  Eugene  Haanel,  Ph.  D.,  Director  of  Mines,  "Recent  Advances  in  the  Con- 
struction of  Electric  Furnaces,"  Ottawa,  1910,  p.  57. 

2  Dr.  K.  G.  Frank  of  the  Siemens  Halske  Co.,  Nov.,  1912. 

3  A  full  account  by  the  inventor  is  given  in  an  appendix  to  Dr.  Haanel's  Euro- 
pean Report,  pp.  165-177.     Also  see  translation  by  P.  McN.  Bennie,  Electro- 
chemical Industry,  vol.  ii,  p.  20. 

4  French  patent,  No.  263,783,  Feb.  6,  1897;  see  European  Report,  p.  166. 


STEEL  FROM  METALLIC  INGREDIENTS  247 


FIG.  108.— Gin's  steel  furnace. 


248  THE  ELECTRIC  FURNACE 

current  through  a  groove  containing  molten  steel.  In  the  Gin  fur- 
nace the  induction  method  is  not  used,  but  the  current  is  led  into 
the  ends  of  the  canal  by  water-cooled  steel  electrodes,  which  enter 
from  below  and  form  part  of  the  furnace  lining.  In  Fig.  io8/  A  is 
the  groove  or  canal  containing  the  molten  steel,  a  portion  of  which, 
as  in  the  Kjellin  furnace,  may  be  left  in  the  furnace  after  each  opera- 
tion to  start  the  current  for  the  next  run,  and  BB  are  the  water-cooled 
steel  terminals.  On  account  of  the  low  resistivity  of  molten  steel, 
the  trough  or  canal  containing  it  should  be  of  great  length  and  small 
cross-section,  in  order  to  avoid  the  use  of  excessively  large  currents. 
This  was  advisable  in  the  Kjellin  furnace,  but  it  is  even  more  neces- 
sary in  the  Gin  furnace,  because  the  current  must  be  developed  in  a 
transformer  and  led  to  the  furnace  by  cables,  all  of  which  are  more 
expensive,  for  equal  power,  as  the  current  is  larger  in  amount,  and 
the  transformer  and  cable  losses  are  also  very  large  when  enormous 
currents  are  employed  at  low  voltages.  In  the  Gin  furnace  the 
trough  A  is  therefore  made  long  and  narrow,  and  in  order  to  secure 
compactness,  with  attendant  economy  of  heat,  it  is  folded  back- 
ward and  forward,  like  the  filament  in  an  incandescent  lamp;  the 
ends,  BBj  being  brought  to  the  same  end  of  the  furnace. 

For  convenience  in  repairing  the  hearth,  it  is  mounted  on  a  car- 
riage which  stands  in  a  furnace  consisting  of  three  walls  and  an 
arched  roof;  the  fourth  side  being  closed  during  the  working  of  the 
furnace  by  a  movable  door.  H  is  one  of  the  two  spouts  through  the 
roof  for  introducing  molten  pig-iron.  The  pig-iron  can  be  converted 
into  steel  by  dilution  with  steel  scrap,  as  in  the  Kjellin  furnace,  or 
by  additions  of  iron-ore,  as  in  the  Heroult  furnace.  When  molten 
pig-iron  is  employed  there  is  no  need  to  leave  any  steel  in  the  furnace 
from  the  previous  run.  The  steel  is  tapped  from  the  furnace  by 
means  of  the  spout,  K;  three  channels,  one  from  each  loop  of  the 
canal,  leading  the  steel  to  the  spout.  The  cables  for  leading  in  the 
current  are  connected  electrically  by  the  bars  GG,  to  the  lower  part 
of  the  water-cooled  terminals  BB.  A  7oo-kw.  furnace  would  have  a 
canal  nearly  30  ft.  long,  9.75  in.  wide  and  19.5  in.  deep;2  and  it 
would  contain  8,550  Ib.  of  steel,  which  would  about  half  fill  the 
groove,  and  would  require  a  current  of  about  50,000  amperes  at  15 
volts. 

The  construction  and  maintenance  of  the  furnace  hearth  will 
probably  be  a  matter  of  considerable  difficulty ;  the  adjacent  branches 

1  Modified  from  figures  in  above  Report. 

2  European  Report,  p.  173. 


STEEL  FROM  METALLIC  INGREDIENTS  249 

of  the  canal  being  near  together,  any  leak  of  metal  from  one  to  the 
next  would  lead  to  a  short-circuiting  of  the  current  and  a  rapid 
enlargement  of  the  leak,  while  the  addition  of  iron-ore  in  the  chan- 
nels will  lead  to  a  corrosion  of  the  walls.  The  best  material  for  the 
construction  of  the  hearth  would  probably  be  chromite,  as  this  is 
very  refractory  and  only  slightly  affected  by  either  silicious  or  irony 
slags. 

The  simple  resistance  furnace  described  above  was  not  found  to 
be  satisfactory,  and  Mr.  Gin  has  modified  its  construction  consider- 
ably, supplying  the  current  by  carbon  electrodes,  or  by  induction, 
in  more  recent  furnaces.1 

Dr.  Bering's  Furnace. — This  furnace,  described  in  Chapter  II  and 
illustrated  in  Fig.  21,  is  a  resistance  furnace  which  can  be  used  for 
steel  making,  and  should,  therefore,  be  referred  to  at  this  point 
although  no  details  of  its  construction  or  use  on  a  large  scale  are 
as  yet  available. 

1  G.  Gin,  'The  Self-circulating  Gin  Furnace  for  the  Electric  Manufacture  of 
Steel,"  Trans.  Am.  Electrochem.  Soc.,  xv,  1909,  p.  205. 


CHAPTER  IX 

THE  PRODUCTION  OF  STEEL  FROM  IRON-ORE 
ELECTRIC  STEEL  SMELTING 

Malleable-iron  or  steel  can  be  produced  by  heating  iron-ore  with  a 
limited  amount  of  carbon;  enough  carbon  being  provided  to  reduce 
the  oxide  of  iron  to  the  metallic  state,  but  not  enough  to  unite  with 
the  reduced  metal  to  make  pig-iron.  The  primitive  metallurgists 
obtained  wrought-iron  and  steel  in  this  manner,  by  reducing  the  ore 
in  small  furnaces,  instead  of  first  making  pig-iron  and  then  turning 
the  pig-iron  into  wrought-iron  or  steel  as  is  the  present  practice. 
Iron  nearly  free  from  carbon  is,  however,  very  difficult  to  melt,  and 
in  the  little  forge  or  furnace  of  the  savage  the  iron  was  not  melted, 
but  obtained  in  the  form  of  a  solid  tump,  which  was  then  cut  up  and 
hammered  into  shape;  it  being  often  necessary  to  pull  the  furnace 
down  in  order  to  extract  the  bloom  of  reduced  iron  or  steel. 

In  modern  times  attempts  have  been  made  to  improve  on  these 
primitive  methods  of  making  steel  from  iron-ore,  and  the  following 
examples  may  be  mentioned:  The  Chenot  process,1  in  which  iron-ore 
mixed  with  charcoal  was  heated  in  a  retort;  the  Siemens  process,2 
in  which  iron-ore  mixed  with  bituminous  coal  was  heated  in  a  revolv- 
ing furnace;  and  the  Husgafvel  process,3  in  which  the  iron-ore  was 
smelted  with  charcoal  in  a  low  blast-furnace.  In  each  case,  the 
product  was  a  lump  of  malleable-iron,  more  or  less  carburized, 
which  was  hammered  into  the  required  shape,  or  could  be  melted 
in  crucibles,  or  in  the  open-hearth  furnace.  With  larger  blast- 
furnaces it  is  possible  to  melt  even  pure  iron,  but  the  melted  iron 
rapidly  absorbs  carbon  from  the  fuel  employed,  and  so  becomes  pig- 
iron.  It  follows  from  this  and  other  reasons,  that  wrought-iron  and 
steel  cannot  be  made  in  a  blast-furnace.  In  the  electric  smelting 
furnace,  however,  the  conditions  are  different,  because,  as  the  heat 
is  supplied  electrically  and  is  not  dependent  upon  the  burning  of  fuel, 
the  amount  of  carbon  supplied  can  bo  adjusted  exactly  to  suit  the 

1  Chenot  process,  F.  W.  Harbord,  "The  Metallurgy  of  Steel,"  1904,  p.  246. 

2  Siemens  process,  F.  W.  Harbord,  "The  Metallurgy  of  Steel,"  1904,  p.  247. 

3  Husgafvel  process,  F.  W.  Harbord,  "The  Metallurgy  of  Steel,"  1904,  p.  246. 

250 


STEEL  FROM  IRON-ORE 


251 


chemical  needs  of  the  ore,  so  as  to  make  a  carbon-free  iron,  or  any 
desired  grade  of  steel. 

Captain  Stassano  has  effected  this  in  his  electric  arc-furnace1 
(Fig.  109),  which  resembles  an  open-hearth  steel  furnace,  in  which 
the  flame  of  burning  gas  has  been  replaced  by  the  flame  of  the 
electric  arc.  The  furnace  consists  of  an  iron  casing  lined  with 
fire-brick,  E,  and  with  an  inner  lining  of  magnesite  bricks,  D.  An 
arc  is  maintained  between  the  ends  G  and  H  of  two  nearly  horizon- 
tal carbon  electrodes,  the  holders  of  which  work  through  air-tight 
stuffing  boxes  in  water-cooled  casings,  /  and  K.  This  arrangement 
prevents  the  escape  of  the  furnace  gases,  cools  the  electrode  holders 
and  prevents  the  oxidation  of  the  external  portions  of  the  electrodes. 


FIG.  109. — Stassano  furnace. 

The  necessary  amount  of  carbon  for  making  iron  or  steel  is  incor- 
porated with  the  ore  in  the  form  of  briquettes,  which  are  introduced 
into  the  furnace,  and  heated  until  the  chemical  reactions  have 
taken  place  and  the  reduced  metal  has  melted.  The  metal  and  slag 
are  then  tapped  out  and  the  operation  is  repeated.  The  carbon 
monoxide,  resulting  from  the  reaction  of  the  carbon  and  the  ore, 
escapes  from  the  furnace  by  the  hole  F.  This  waste  gas  might  be 
employed  for  drying  and  preheating  the  ore. 

Dr.  Haanel  was  unable  to  see  Stassano's  furnace  at  Turin  in 
operation,  as  it  was  out  of-  repair  at  the  time  of  his  visit,  but  he  gives 
a  description  of  the  furnace  and  prints  an  account  of  the  process 

1  Electrochem.  Industry,  vol.  i,  pp.  247,  363,  and  461;  vol.  iii,  p.  391. 
Engineering  and  Mining  Journal,  June  15,  1907,  p.  1135. 


252  THE  ELECTRIC  FURNACE 

written  by  the  inventor.1  The  newer  forms  of  furnace  are  inclined 
about  7°  from  the  vertical  and  rotate  slowly  round  this  inclined  axis, 
with  a  view  to  stirring  up  the  charge  and  allowing  the  heat  of  the 
arc  to  act  more  freely  on  the  ore.  In  some  furnaces  three  electrodes 
are  used,  with  three-phase  current,  while  in  other  furnaces  four 
electrodes  are  employed.  Stassano  gives  the  following  particulars 
with  regard  to  a  furnace  of  1,000  h.p.2  The  cost  of  the  furnace  is 
$5,000,  the  output  per  day  is  4  or  5  tons,  a  current  of  4,900  amperes 
at  150  volts  is  distributed  to  four  electrodes  (2,450  amperes  to  each 
electrode).  The  electrodes  are  6  in.  in  diameter  and  4  ft.  to  5  ft. 
long.  A  5-ft.  electrode  weighs  130  lb.,  and  costs  3  cents  a  pound. 
The  consumption  of  electrodes  is  22  to  33  lb.  per  ton  of  product,  that 
is  70  cents  to  $i  per  ton  of  steel.  The  lining  is  of  magnesite  bricks, 
and  two  days  are  required  for  repairing  the  furnace.  The  lining 
will  last  at  least  40  days.  One  man  is  needed  per  furnace  to  regulate 
the  arc;  one  man  for  charging  two  furnaces,  and  five  men  for  tapping 
six  furnaces.  Taking  the  above  figures  of  i,ooO'E.H.P.  days  for 
4  or  5  tons  of  iron  or  steel,  each  ton  would  need  0.55  to  0.69  h.p.- 
years  for  its  production.  Dr.  Goldschmidt3  investigated  the  process 
in  1903  on  behalf  of  the  German  patent  office,  and  found  that  it  was 
technically  successful,  making  workably  ductile  iron  with  less  than 
0.2  per  cent,  of  carbon  directly  from  pure  Italian  ores.  The  energy 
used  was  0.46  to  0.49  h.p.-years  per  metric  ton  of  iron.  The  process 
was  reported  as  too  expensive  to  compete  with  existing  methods  in 
Germany. 

A  later  series  of  experiments  was  made  by  Stassano  in  a  three- 
phase  furnace  of  150  kw.4 

Particulars  of  some  of  these  experiments  are  given  in  the  following 
table. 

TABLE  XVII.— STASSANO'S  EXPERIMENTS  IN  1908 

Ore 

Fe2O3 68 .  70  per  cent.          CaO i .  oo  per  cent. 

Mn3O4 3.23percent.          MgO 5.67percent. 

SiO2 17.15  percent.          P o.ispercent. 

A12O3 2.oopercent.          S o.i2percent. 

Charge 

Ore 100  kg.          25    per    cent,    solution    of 

Limestone 35  kg.  sodium  silicate 8  kg. 

Charcoal 24  kg.          Calcium  carbide 5  kg. 

TDr.  Haanel,  European  Report,  1904,  pp.  178-214. 

2  Dr.  Haanel,  European  Report,  1904,  p.  12. 

3  Electrochemical  Industry,  i,  1903,  p.  247. 

4  Electrochem.  and  Metal.  Ind.,  vol.  vi,  1908,  p.  315;  vol.  ix,  1911,  p.  642. 


STEEL  FROM  IRON-ORE 


253 


Products 


Number  of  test  

i 

II 

III 

IV 

Carbon 

Per  cent. 

O    2< 

Per  cent, 
o.  26 

Per  cent. 

o.  30 

Per  cent. 
0.80 

Manganese          

O.  12 

O.  21 

o.  24 

0.30 

Silicon 

O.O7 

O.CK 

o.  14 

O.  22 

Phosphorus  

O.OIO 

O.OIO 

0.015 

0.015 

Sulphur                           .  . 

o  .o6< 

0.040 

0.07 

0.045 

Kw.-hours  per  ton  

0.51 

0.49 

0.46 

0.48 

In  these  experiments  the  ore,  limestone  and  charcoal  were  crushed, 
mixed  and  briquetted  with  a  25  per  cent,  solution  of  water-glass. 

Comparing  the  direct  process  of  Stassano  with  the  more  usual 
plan  of  smelting  first  to  pig-iron,  and  then  refining  the  iron  and  mak- 
ing steel;  it  will  be  seen  that  the  electrical  energy  needed  to  smelt 
ore  directly  to  steel  in  the  Stassano  furnace  is  greater  than  the  energy 
needed  for  the  other  two  processes,  and  that  his  process  was  used 
with  nearly  pure  ores,  while  the  indirect  method  allows  the  use  of 
any  kind  of  iron-ore.  The  Stassano  furnace  is  intermittent  in  action, 
as  each  charge  of  ore  must  be  reduced,  melted,  refined  and  tapped 
before  a  fresh  one  can  be  introduced.  The  economy  of  heat  is 
poor  because  the  heat  of  the  escaping  gas  is  not  utilized,  and  its 
chemical  energy  is  not  employed,  as  it  might  be,  for  the  reduction 
or  preheating  of  the  ore. 

With  regard  to  the  possibility  of  producing  pure  steel  in  a  single 
operation  from  impure  ores,  the  conditions  under  which  the  hurtful 
elements,  sulphur  and  phosphorus,  are  removed  from  iron  and  steel 
may  be  considered.  In  the  blast-furnace,  sulphur  is  removed  from 
the  iron  and  passes  into  the  slag  as  calcium  sulphide,  its  removal  in 
this  way  being  more  complete  as  the  furnace  is  more  strongly  reduc- 
ing and  the  slag  is  richer  in  lime;  that  is,  when  the  conditions  are 
favorable  for  the  formation  of  calcium  to  combine  with  the  sulphur. 
The  electric  furnace  making  pig-iron  has  more  strongly  reducing 
conditions,  and  can  carry  more  lime  in  the  slag  than  is  possible  in 
the  blast-furnace;  this  explains  its  superior  ability  to  eliminate 
sulphur.  When,  however,  the  ore  is  smelted  directly  to  steel  in  the 
electric  furnace,  the  conditions  are  far  less  reducing,  and  there  is 
less  reason  for  expecting  the  removal  of  sulphur  as  calcium  sulphide, 
even  in  the  presence  of  a  limey  slag.  The  sulphur  remaining  in  the 
steel  after  the  smelting  operation  can,  however,  be  removed  very 


254  THE  ELECTRIC  FURNACE 

perfectly  in  the  electric  arc-furnace  by  means  of  a  limey  slag  to 
which  carbon  has  been  added. 

With  regard  to  the  elimination  of  phosphorus  the  conditions- are 
quite  the  reverse,  as  this  element  can  only  be  removed  by  oxidation. 
In  the  blast-furnace  any  phosphorus  in  the  charge  finds  its  way  into 
the  pig-iron,  and  the  same  takes  place  in  the  electric  furnace  making 
pig-iron;  but  in  the  open-hearth  furnace,  with  a  strongly  basic  slag, 
the  removal  of  phosphorus  can  be  satisfactorily  accomplished,  and 
the  same  will  hold  good  in  the  production  of  steel  directly  from  the 
ore  in  the  electric  furnace,  if  the  slag  is  limey  and  sufficiently 
oxidizing. 

In  an  electric  steel-making  furnace  such  as  the  Heroult  or  the 
Stassano,  in  which  the  molten  steel  can  be  washed  by  the  repeated 
addition  and  removal  of  limey  slags,  any  sulphur  and  phosphorus 
can  ultimately  be  removed;  but  the  production  of  steel  directly 
from  the  ore  can  be  accomplished  most  economically  in  some  form 
of  shaft  furnace,  that  is  a  furnace  resembling  the  Heroult  ore-smelt- 
ing furnace,  which  is  operated  continuously,  instead  of  intermittently 
like  the  steel  furnaces.  It  was  therefore  a  question  of  the  greatest 
importance  in  regard  to  the  possible  production  of  steel  directly 
from  the  ore,  to  determine  whether  in  a  continuous  smelting  furnace, 
steel  free  from  sulphur  and  phosphorus  could  be  produced  from  ores 
carrying  the  usual  proportions  of  these  elements.  The  author 
accordingly  proposed  the  problem  to  two  of  his  students,  Messrs. 
W.  G.  Brown  and  F.  E.  Lathe,  and  embodied  the  results  of  their 
work  in  a  paper  read  before  the  Canadian  Mining  Institute  in  March, 
1907.  The  experiments  were  made  in  a  small  shaft  furnace  resem- 
bling the  furnace  used  at  Sault  Ste.  Marie,  but  lined  with  burnt  mag- 
nesite.  As  no  carbon  could  be  used  as  a  lining  for  the  crucible  of 
the  furnace,  electrical  connection  was  made  by  means  of  an  iron 
rod  passing  through  the  bottom  of  the  furnace.  The  power  available 
was  rather  small,  but  it  was  found  possible  to  run  the  furnace  regu- 
larly for  a  few  hours  at  a  time,  producing  low-carbon  steel  of  which 
some  2  or  3  Ib.  were  tapped  at  intervals  of  about  half  an  hour. 

The  ore  used  was  a  pure  hematite  from  Lake  Superior,  containing 
97  per  cent,  of  ferric  oxide,  2.23  per  cent,  of  silica,  and  0.68  per  cent, 
of  alumina.  Clay,  sand,  and  lime  were  added  to  make  a  slag  equal 
to  about  half  the  weight  of  the  resulting  metal,  and  i  per  cent, 
each  of  sulphur  and  phosphorus  was  added  in  the  form  of  mono- 
sulphide  of  iron  and  calcium  phosphate. 

Analyses  of  the  steel  and  slag  from  a  number  of  the  taps  are  given 


STEEL  FROM  IRON-ORE 


255 


in  Table  XVIII,  and  show  very  clearly  the  effect  of  lowering  the 
carbon  in  the  charge,  and  so  producing  steel  instead  of  pig-iron. 
If  sufficient  carbon  had  been  added  in  the  charge,  a  pig-iron  would 
have  been  produced  rich  in  carbon  and  silicon,  low  in  sulphur, 
and  with  more  than  i  per  cent,  of  phosphorus.  With  the  smaller 
amount  of  carbon  which  was  charged  in  these  experiments,  the  result- 
ing iron  contained  less  carbon  and  silicon  and  more  sulphur  (see 
No.  i  in  the  table).  As  the  carbon  in  the  charge  was  diminished, 
the  resulting  metal  contained  still  less  carbon  and  silicon,  and  at 
the  same  time  the  phosphorus  in  the  steel  was  progressively  reduced, 
until  in  the  lowest  carbon  steels  the  phosphorus  became  low  enough 
for  structural  purposes.  The  sulphur,  on  the  other  hand,  which 
would  have  been  nearly  eliminated  in  the  production  of  pig-iron, 
increased  with  the  decrease  of  carbon,  no  doubt  because  there  was 
less  opportunity  for  its  removal  as  calcium  sulphide;  but  further 
decrease  of  carbon,  resulting  in  a  highly  oxidized  slag,  served  to 
remove  a  portion  of  the  sulphur,  probably  a  calcium  sulphate,  in 
the  same  way  that  it  is  removed  in  the  basic  open-hearth  furnace. 

TABLE  XVIII.— STEEL  AND  SLAG  ANALYSES 


Test  No. 

i 

2 

3 

A 

5 

6 

7 

Steel 
C.,  per  cent  

2    OQ 

i  16 

O    54 

o  088 

o  088 

o  088 

O    OQI 

Si.   per  cent 

O    2O 

o  i  < 

O    24 

S.,  per  cent  

o.  75 

O.  QI 

I  .04 

O    54 

o  65 

o  68 

o  47 

P.,  per  cent. 

O   40 

o  24 

O    2O 

O    O3Q 

o  046 

o  081 

o  031 

Slag 
FeO.,  per  cent  
SiOz,  per  cent  

4-'5 
31  .  7 

7-i 
30.  3 

3-6 

^2  .  2 

20.54 

16.  77 

20.64 
18.02 

26.94 
15.66 

33-46 

15    42 

CaO,  per  cent  

30   7 

to  8 

36  o 

27  07 

•21:    17 

•2<r    17 

32    <J4 

MgO  per  cent 

i?  8 

21    ^ 

17  6 

22    34 

1  3    OO 

13  84 

1  1    34 

Al2Os.  ner  cent.  .  . 

13.  2 

O.  3 

II  .  7 

7.48 

S.30 

3.  <?3 

2.  13 

The  first  three  analyses  are  taken  from  one  run  of  the  furnace, 
while  the  last  four  are  from  another  run,  in  which  less  carbon  was 
charged.  The  second  run  appears  to  show  that  the  carbon  in  the 
steel  could  be  lowered  to  about  0.09  per  cent.,  but  that  any  further 
reduction  in  the  amount  of  carbon  charged,  merely  increased  the 
already  large  percentage  of  iron  oxide  in  the  slag,  without  lowering 
any  further  the  carbon  in  the  steel. 

While  these  analyses  only  represent  the  result  of  smelting  an  iron- 
ore  in  an  electric  furnace  with  particular  conditions  of  charge,  shape 
of  furnace,  current  density,  etc.,  and  changes  in  any  of  these  conditions 


256  THE  ELECTRIC  FURNACE 

might  influence  the  composition  of  the  resulting  steel,  they  indicate 
that  in  the  electrothermic  production  of  steel  directly  from  a  sulphur- 
ous ore,  it  will  not  be  easy  to  remove  the  sulphur  in  an  electric  fur- 
nace operating  continuously  like  a  blast-furnace;  although  this  is 
possible  with  intermittent  operation,  as  in  an  electric  open-hearth 
furnace.  Phosphorus,  on  the  other  hand,  can  be  satisfactorily  re- 
moved when  low- carbon  steel  is  produced. 

In  smelting  iron- ores  to  obtain  a  low- carbon  product,  the  carbon 
electrodes,  if  in  contact  with  the  slag  or  melting  ore,  will  be  liable 
to  more  rapid  corrosion  than  when  smelting  for  pig-iron;  on  account  of 
the  scarcity  of  carbon  in  the  charge.  This  difficulty,  if  it  were  found 
to  be  serious,  might  be  overcome  by  the  use  of  a  furnace  like  that  of 
de  Laval,  Fig.  20,  in  which  the  reduced  and  melted  metal,  collecting 
in  two  troughs,  serves  as  the  electrodes;  electrical  contact  being  made 
with  the  molten  metal  by  solid  rods  of  the  same  material.  Another 
plan  for  avoiding  the  use  of  carbon  electrodes  is  to  employ  the  induc- 
tion principle,  as  in  the  Snyder  induction  furnace,  Fig.  131,  or  in 
some  Swedish  ore-smelting  furnaces1  which  have  a  shaft  for  the  reduic- 
tion  of  the  ore,  while  the  molten  pig-iron,  resulting  from  the  opera- 
tion, collects  in  an  annular  channel  where  it  is  heated  by  an  induced 
electric  current.  The  cost  of  producing  low-carbon  steel  direct  from 
pure  Italian  ore,  in  the  Stassano  furnace,  has  been  estimated  by  Dr. 
Goldschmidt,  who  sets  the  cost  of  a  ton  of  such  steel  at  $18.80. 
The  furnace  does  not  utilize  the  heat  of  the  current  very  perfectly, 
and  with  improved  furnaces  and  better  conditions  for  the  purchase 
of  general  supplies,  a  lower  figure  might  be  expected. 

In  the  year  1905  Mr.  J.  W.  Evans  of  Belleville,  Canada,  took  up 
the  direct  production  of  steel  in  the  electric  furnace  from  titaniferous 
and  sulphurous  ores;  and  in  March,  1906,  he  showed,  at  the  meeting 
of  the  Canadian  Mining  Institute,2  a  number  of  small  tools  made 
from  this  steel.  The  following  table  shows  the  extent  to  which  the 
sulphur  and  titanium  were  eliminated  in  these  experiments. 

The  ore  was  smelted  in  a  small  arc-furnace,  which  was  operated 
intermittently,  so  that  the  steel  and  slag  were  superheated  after  the 
ore  charge  was  completely  smelted.  In  smelting  the  titaniferous  ore, 
the  addition  of  lime  in  the  charge  will  remove  the  titanium.  If  little 
lime  is  used,  in  order  to  retain  some  titanium  in  the  steel,  the  silicon 
will  also  be  high.  Mr.  Evans  continued  his  work  on  titaniferous  ores 

1Dr.  Haanel,  1907,  Report,  p.  104. 

2  J.  W.  Evans,  Jour.  Can.  Min.  Inst.,  vol.  ix,  1906,  p.  128.  Mr.  Evans  has 
patented  a  titanium  steel  and  the  method  of  making  it  from  titaniferous  ores. 


STEEL  FROM  IRON-ORE 


257 


TABLE  XIX.— STEEL    FROM 
Coe  Hill  Ore 
(Sulphurous) 


TITANIFEROUS    AND    SULPHUROUS 
Bowen  Mine  Ore 
(Titaniferous) 


ORES 


Or 
Iron 

e 

68  c 

>i% 

Ore 
Iron  

AC 

[7% 

Sulphur 

I    C 

>i% 

Titanium 

7   , 

14% 

Test  No 

I 

II 

III 

Test  No  • 

I 

ii 

III 

Steel 
Carbon 

o  05% 

0.07% 

0.06% 

Steel 
Carbon  

o.  51% 

0.84% 

0.87% 

Silicon 

O    OI% 

Trace 

o  04% 

Silicon.     .    . 

o  62% 

2    31% 

o  o«;% 

Sulphur  

0.12% 

0.17% 

0.08% 

Titanium.  .  .  . 

o.37% 

1.02% 

None 

during  the  years  1908  and  1909,  producing  steel  in  quantities  of  a  few 
pounds  at  a  time,  from  which  he  made  tools  of  very  good  quality. 

Mr.  Evans'  experimental  furnace  consisted  of  a  small  chamber, 
open  at  the  top,  and  provided  with  a  pair  of  lateral  electrodes  which 
at  the  commencement  of  the  process  were  nearly  horizontal;  an  arc 
being  formed  between  them  as  in  the  Stassano  furnace.  When  the 
ore  charge  was  nearly  all  smelted,  the  electrodes  were  depressed,  so 
that  their  ends  entered  or  nearly  touched  the  slag;  the  electric  current 
then  passing  directly  through  the  slag,  as  in  the  Heroult  steel  furnace. 
On  a  larger  scale  the  same  effect  would  be  obtained  by  the  use  of  two 
pairs  of  electrodes,  as  in  the  laboratory  furnace,  Fig.  72;  only  one  pair 
being  used  at  once. 

During  the  summer  of  1909  the  author  visited  Mr.  Evans'  plant, 
and  suggested  a  number  of  modifications  in  the  furnace  and  process 
with  a  view  to  greater  economy  in  operation.  An  intermittent  fur- 
ance,  like  an  open-hearth,  was  apparently  necessary  for  finishing  the 
steel,  but  a  shaft  furnace  would  be  more  efficient  in  smelting  the  ore. 
The  author  suggested  that  these  two  elements  should  be  combined  as 
in  the  furnace  shown  in  Fig.  no. 

In  this  furnace1  the  ore  charge  is  heated  and  partly  reduced  to  metal 
in  the  vertical  retorts,  R.  It  is  then  fed  mechanically  into  the  elec- 
tric smelting  furnace  where  it  melts.  When  sufficient  charge  has 
been  introduced,  the  feeding  mechanism  is  stopped,  and  after  the 
remainder  of  the  charge  has  melted,  part  of  the  slag  is  tapped  out  and 
the  steel  and  the  remaining  slag  are  superheated  before  tapping. 
Any  additions  that  are  needed  can  be  made  at  this  time.  The  car- 
bon monoxide,  formed  in  the  electric  furnace,  passes  up  the  shaft 

*A.  Stansfield,  "Tool  Steel  Direct  from  the  Ore  in  an  Electric  Furnace," 
Jour.  Can.  Min.  Inst.,  xiii,  1910,  p.  151. 
17 


258 


THE  ELECTRIC  FURNACE 


through  the  ore,  thus  helping  in  its  reduction  to  metal,  and  finally 
escapes  into  the  chamber  around  the  retorts  where  it  burns;  air  for 
its  combustion  being  admitted  through  holes,  H,  in  the  walls. 

The  experiments  were  continued  by  Mr.  Evans  at  Belleville,  and 
by  the  author  and  Mr.  C.  G.  Porter  in  the  Metallurgical  laboratories 


FIG.  no. — Evans-Stansfield  furnace. 

of  McGill  University,  using  the  furnace  of  Fig.  72,  as  the  combination 
open-hearth  and  shaft  furnace  was  not  convenient  to  operate  on  a 
small  scale. 

The  process  at  first  consisted  in  crushing  the  ore,  mixing  it  with 
lime  and  charcoal,  and  briquetting  the  mixture  with  tar  and  pitch. 
The  briquets  were  heated  in  closed  chambers  to  drive  off  the  volatile 


STEEL  FROM  IRON-ORE  259 

matter,  and  to  reduce  the  iron  to  metal,  after  which  the  red-hot 
briquets  were  fed  into  an  electric  furnace  and  melted  down,  yielding 
steel  and  slag.  The  closed  chambers  for  heating  the  briquets  would 
in  practice  consist  of  towers  or  vertical  retorts  heated  externally  by 
burning  the  gases  derived  from  the  charge.  In  view  of  the  expense  of 
briquetting,  it  is  intended  to  dispense  with  this  part  of  the  process, 
and  to  smelt  the  ore,  crushed  roughly,  and  mixed  with  the  lime  and 
charcoal.  The  results  obtained  so  far  have  been  with  the  original 
briquetting  process  and  are  sufficiently  good  to  warrant  a  test  on  a 
small  working  scale  such  as  is  now  being  made.  The  expense  of  bri- 
quetting, while  undesirable,  would  not  be  prohibitive  in  the  pro- 
duction of  tool  steel,  if  it  were  found  necessary  for  the  process. 

The  ore  mostly  used  in  the  experiments  was  obtained  from  the 
Orton  Mine,  Hastings  County,  Ontario,  Canada.  A  shipment  of 
this  gave  the  following  analysis: 

Magnetic  oxide 75.40%  (iron  54%)  Sulphur Trace 

Titanium  oxide 12.65%  Phosphorus 0.015% 

Silica 1.5%  Nickel 0.12% 

Lime 5-75%  Vanadium Trace 

Alumina 3-95% 

A  suitable  charge  consisted  of  100  parts  of  ore,  12  parts  of  charcoal, 
8  parts  of  lime,  7  parts  of  pitch,  and  7  parts  of  tar.  The  mixture 
was  warmed  and  briquetted,  and  the  briquets  were  heated  in  covered 
crucibles,  or  in  an  iron  box,  for  about  five  hours,  to  a  temperature  of 
about  900°  C.  The  iron  oxide  in  the  ore  briquets  was  all  reduced  to 
ferrous  oxide  and  a  variable  quantity,  which  might  average  about  50 
per  cent.,  had  been  reduced  to  the  metallic  state  by  the  operation  of 
heating  in  closed  crucibles.  With  a  hematite  ore  it  would  be  easy  to 
obtain  an  almost  perfect  reduction  to  metal  in  the  preliminary  heat- 
ing, and  this  would  greatly  reduce  the  consumption  of  electrical 
energy  in  the  smelting  operation. 

The  baked  briquets  were  smelted  in  the  furnace  shown  in  Fig.  72. 
After  the  charge  was  all  melted  the  heating  was  continued  for  about 
an  hour  to  superheat  the  steel  and  make  it  sound.  The  slag  was  then 
decanted  by  tilting  the  furnace  and  the  metal  poured  into  a  ladle  and 
from  that  into  molds.  The  steel  varied  considerably  in  its  carbon 
content,  and 'retained  a  few  tenths  of  a  per  cent. -  of  titanium.  The 
ingots  were  usually  sound  and  when  cut  into  bars,  sharpened  and 
tempered,  could  be  used  directly  as  lathe  tools.  These  tools  were 
found  to  cut  better  than  tools  of  good  carbon  steel,  and  sometimes 


260  TEE  ELECTRIC  FURNACE 

as  well  as  expensive  "quick-cutting'*  steel.  The  tools  were  usually 
cut  from  the  ingot,  or  made  by  pouring  the  steel  into  molds  of  the 
desired  shape,  as  the  steel  was  frequently  too  hard  for  forging;  but 
an  ingot  of  steel  with  0.75  per  cent,  carbon  was  drawn  down  success- 
fully under  the  hammer.  When  smelting  the  titaniferous  ore  it  was 
not  necessary  to  add  any  ferro-alloy  in  the  furnace  or  ladle,  as  the 
titanium  ensured  a  sound  product,  but  in  smelting  a  pure  magnetite 
ore,  the  usual  additions  of  ferro-manganese  and  ferro-silicon  were 
made  in  the  furnace.  Using  a  mixture  of  100  parts  pure  magnetite, 
30  parts  titaniferous  magnetite,  15.5  parts  charcoal,  7  parts  pitch,  7 
parts  tar,  and  8  parts  of  lime,  a  run  of  4  1/2  hours,  using  23  kw.,gave 
57  1/2  Ib.  of  steel,  corresponding  to  0.46  kw.  years  per  long  ton,  and  a 
second  run  of  3  hours  using  25  kw.  yielded  41  Ib.  of  steel.  The 
steel  contained  1.55  per  cent,  carbon,  0.16  per  cent,  of  manganese  and 
0.65  per  cent,  of  titanium.  The  electrode  loss  was  equal  to  17  Ib.  per 
long  ton  of  steel. 

In  starting  the  furnace  an  arc  was  struck  between  the  end  electrodes 
and  when  a  sufficient  quantity  of  molten-  metal  and  slag  had  been 
formed,  these  electrodes  were  withdrawn,  and  the  vertical  electrodes 
used  during  the  remainder  of  the  heat. 

Another  method  was  to  use  the  vertical  electrodes  during  the  whole 
run,  a  short  piece  of  electrode  being  laid  between  their  ends  for  start- 
ing the  arc.  The  electrodes  were  of  graphite,  1.5  in.  in  diameter, 
being  somewhat  larger  and  closer  together  than  those  shown  in  the 
figure.  The  furnace  was  heated  as  strongly  as  possible  by  means  of  a 
blowpipe,  before  each  test.  The  briquets  were  charged  into  the  fur- 
nace through  holes  in  the  roof. 

The  ores  used  were  low  in  phosphorus  and  sulphur,  and  the  tests 
did  not  throw  much  light  on  the  elimination  of  these  elements.  The 
titanium  in  the  ore  served  to  remove  oxygen  and  possibly  nitrogen 
from  the  steel,  thus  giving  a  sound  product,  and  the  small  proportion 
of  titanium  that  entered  the  steel  gave  additional  hardness.  The 
amount  of  titanium  in  the  steel  could  be  controlled  by  varying  the 
lime  in  the  charge;  a  basic  slag  removing  more  titanium  than  an  acid 
one.  The  experiments  were  decidedly  encouraging,  but  the  power 
available  was  insufficient,  and  the  furnace  could  not  be  operated 
continuously.  A  plant  has  been  built  at  Belleville  for  testing  the 
process  on  a  small  working  scale;  the  first  furnace  having'an  output  of 
i  ton  of  tool  steel  per  24  hours. 

For  the  production  of  tool  steel  the  cost  of  the  process  would  be 
sufficiently  low  even  if  briquetting  were  found  desirable,  and  without 


STEEL  FROM  IRON-ORE 


261 


any  economies  such  as  would  be  obtained  by  the  use  of  a  shaft  fur- 
nace. Dispensing  with  briquetting  and  assuming  a  reasonable  in- 
crease in  economy  to  result  from  the  particular  design  and  larger 
size  of  the  furnace,  steel  could  be  produced  commercially  for  making 


^f'fff/ffff//////f//////f/f//ff///f////f//ff/f/ff/////f/f/f// 
FIG.  in. — Keeney  steel  furnace. 

steel  castings,  and  possibly  for  structural  purposes  in  competition 
with  open-hearth  or  Bessemer  steel  refined  in  the  electric  furnace. 

Some  experiments  on  the  production  of  steel  from  iron  ore  in  the 
electric  furnace  have  recently  been  made  by  R.  M.  Keeney,1  at  the 

1  Keeney,  Trans.  Iron  and  Steel  Inst.,  1912,  vol.  iv,  p.  108. 


262  THE  ELECTRIC  FURNACE 

Colorado  School  of  Mines,  as  a  Carnegie  scholarship  research  for  the 
Iron  and  Steel  Institute.  The  furnace,  Fig.  in,  was  rectangular  in 
plan,  4  in.  by  9  in.  at  the  bottom,  and  about  8  in.  by  12  in.  at  the  top, 
with  a  height  of  12  in.  The  furnace  was  lined  with  a  4  i/2-in.  course 
of  magnesite-brick  with  an  inner  lining  of  magnesite  paste.  Two 
graphitized  electrodes  entered  through  the  top  of  the  furnace  and 
were  connected  to  one  pole  of  the  electrical  supply;  the  return  elec- 
trode being  composed  of  iron  rods  embedded  in  magnesite,  which 
form  the  bottom  of  the  furnace.  The  graphite  electrodes  were  6  in. 
by  1/2  in.  in  section;  this  shape  being  selected  with  a  view  to  a 
better  distribution  of  the  current.  The  electric  power  available  was 
limited  to  about  10  kw.  of  i2$-cycle  alternating  current  at  about  55 
volts.  The  power  was  taken  from  the  ammeter  and  voltmeter 
readings,  with  an  assumed  power-factor  of  80  per  cent. 

The  ore  was  a  hematite  containing  57  per  cent,  of  iron,  9.25  per 
cent,  of  silica,  0.12  per  cent,  of  phosphorus  and  0.14  per  cent,  of  sul- 
phur. The  coke  contained  81  per  cent,  of  fixed  carbon,  18  per  cent, 
of  ash  and  0.54  per  cent,  of  sulphur.  A  charge  for  soft  steel  consisted 
of  6.82  kg.  of  ore,  1.02  kg.  of  coke  and  3.66  kg.  of  limestone.  The 
operation  lasted  for  one  hour  and  40  minutes  with  an  average  power 
of  9  kw.;  ten  minutes  being  spent  in  heating  the  empty  furnace,  one 
hour  and  ten  minutes  in  smelting  the  charge,  and  20  minutes  in 
heating  the  reduced  charge  before  tapping.  The  steel  reduced 
amounted  to  3.30  kg.,  but  only  1.22  kg.  was  tapped  as  the  rest  froze 
in  the  furnace.  The  steel  contained  0.08  per  cent,  of  carbon,  0.17 
per  cent,  of  manganese,  0.07  per  cent,  of  silicon,  0.088  per  cent,  of 
phosphorus,  and  0.027  per  cent,  of  sulphur.  0.51  of  a  kilowatt-year 
was  employed  per  (metric)  ton  of  steel  reduced,  and  the  electrode 
consumption  was  18.2  kg.  per  ton  of  steel.  Eighty-five  per  cent,  of 
the  iron  in  the  charge  was  reduced  and  formed  steel.  Twenty  runs 
were  made,  yielding  steel  which  varied  from  0.08  per  cent,  to  2.25  per 
cent,  of  carbon.  The  phosphorus  varied  from  o.oi  per  cent,  to 
0.12  per  cent.,  with  an  average  of  0.059  Per  cent.,  and  the  sulphur 
from  0.02  per  cent,  to  0.14  per  cent.,  with  an  average  of  0.059  Per 
cent.  An  average  of  77  per  cent,  of  the  total  phosphorus  and  87  per 
cent,  of  the  total  sulphur  in  the  charge  was  eliminated  in  the  different 
runs. 

In  regard  to  these  results  it  may  be  mentioned  that  a  further 
removal  of  phosphorus  and  sulphur  could  easily  be  effected,  on  the 
large  scale,  by  the  use  of  suitable  slags  in  a  refining  operation  at  the 
conclusion  of  the  smelting  process.  On  the  other  hand,  as  the  heat- 


STEEL  FROM  IRON-ORE  263 

ing  was  continued  in  these  experiments  for  some  time  after  the  smelt- 
ing was  finished,  the  elimination  mentioned  may  be  better  than  would 
be  obtained  in  a  furnace  that  was  operating  continuously,  like  a 
shaft  furnace.  The  consumption  of  electrical  energy  per  ton  of 
metal  reduced  varied  from  0.44  to  i.n  with  an  average  of  0.57  kw.- 
years,  and  the  electrode  consumption  varied  from  18  to  67,  with  an 
average  of  41  kg.  per  ton  of  steel  reduced.  In  a  large  furnace  these 
figures  could  probably  be  reduced  to  0.30  kw.-years  and  15  to  20  kg. 
of  electrodes  per  ton  of  steel.  Keeney  calculates  that  0.22  kw.- 
years  are  theoretically  necessary  for  the  production  of  i  ton  of  steel. 

The  iron  lost  in  the  slags  in  the  various  runs  was  6.1  per  cent,  of  the 
iron  charged  and  should  not  exceed  4  per  cent,  when  operating  on  a 
large  scale.  The  direct  production  of  steel  from  iron-ore  has  been 
tested  at  the  plant  of  "La  Neo-Metallurgie"  in  France  on  a  larger 
scale,1  using  an  arc- furnace  of  120  kw.  Various  ores  were  used  with 
coke,  charcoal  and  anthracite  as  reducing  materials.  The  ore  was 
first  crushed  and  briquetted  in  admixture  with  the  coal  or  charcoal; 
later  the  ore  was  briquetted  alone  and  charged  with  charcoal,  and 
finally,  the  ore  and  fuel  were  crushed,  mixed  and  charged  without 
briquetting.  The  results  were  the  same  in  each  case,  except  that 
the  rate  of  smelting  became  somewhat  less  in  the  latter  cases, 
being,  for  the  same  power,  35  kg.,  32  kg.,  and  31  kg.  of  steel  per  hour, 
respectively. 

Steel  was  obtained,  both  low  and  high  in  carbon,  having  very  little 
sulphur  and  phosphorus,  showing  a  satisfactory  elimination  of  these 
elements.  It  is  stated  that  no  refining  operation  was  employed  but 
the  furnace  was  of  the  intermittent  type,  so  that  the  steel  and  slag 
would  be  heated  together  in  the  furnace,  after  the  ore  was  all  reduced. 
The  reduction  of  hematite  with  charcoal,  to  form  soft  steel,  required 
3,430  kw.-hours  per  metric  ton  of  steel,  and  it  was  estimated  that 
2,6ookw.-hours  would  suffice  in  a  2oo-kw.  furnace  and  2,500  kw.- 
hours  or  less  in  a  very  large  furnace. 

SUMMARY  OF  ELECTRIC  STEEL  SMELTING 

1.  Steel  can  and  probably  will  be  made  directly  from  iron-ore. 

2.  The  process  will  consist  of  a  reducing  operation  followed  by 
a  refining  operation. 

3.  The  refining  operation  is  needed  partly  for  the  more  perfect 
removal  of  impurities  such  as  phosphorus,  sulphur  and  dissolved 

1  Revue  de  Metallurgie,  Dec., -1910,  p.  1190. 


264  THE  ELECTRIC  FURNACE 

gases,  and  partly  to  adjust  the  proportion  of  carbon,  silicon,  mangan- 
ese and  other  desirable  elements. 

4.  The  reduction  will  be  effected  in  some  form  of  shaft  furnace. 

5 .  The  refining  will  be  effected  in  some  form  of  electric  open- hearth 
furnace,  or  in  a  ladle. 

6.  The  furnaces  for  reducing  and  refining  may  be  separate,  the 
metal  being  transferred  from  one  to  the  other,  or  they  may  be 
combined. 

7.  In  a  combination  furnace  the  ore  must  be  held,  periodically, 
in  the  reducing  shaft,  while  the  accumulated  steel  is  refined. 

8.  In  smelting  iron-ore  to  make  steel,  phosphorus  and  sulphur  in 
the  charge  can  be  largely  eliminated  from  the  steel.     The  removal 
of  phosphorus  is  more  perfect,  and  that  of  sulphur  less  perfect,  than 
in  smelting  for  pig-iron. 

9.  In  consequence  of  the  less  perfect  removal  of  sulphur  the  direct 
process  should  be  restricted  to  ores  and  fuels  that  are  reasonably 
low  in  sulphur. 

10.  In  consequence  of  the  better  removal  of  phosphorus,  the 
oxidizing-refining  process,  usually  employed  in  steel-making,  can 
often  be  omitted  in  the  direct  production  of  steel,  and  there  is  there- 
fore no  need  to  remove  the  carbon,  and  then  return  it  again  to  the 
steel. 

11.  In  the  electric  reduction  furnace  (for  pig-iron  or  for  steel), 
the  simple  shaft  of  the  blast-furnace  should  be  modified  for  greater 
economy  in  view  of  the  absence  of  an  air-blast,  and  the  consequent 
lower  temperature  of  the  ore  in  the  shaft. 

12.  This  modification  may  consist  of  a  system  of  gas  circulation, 
as  in  the  Swedish  iron  furnaces,  or  of  a  system  of  retorts  for  heating 
the  charge,  as  in  the  Evans-Stansfield  steel  furnace. 

Note. — The  author  believes  that  the  above  statements  represent 
generally  the  conditions  necessary  for  the  direct  production  of  steel 
in  the  electric  furnace.  It  is  possible  that  with  some  particular  ore, 
or  combination  of  ores,  steel  of  suitable  chemical  composition  and 
mechanical  properties  may  result  from  the  smelting  operation  with- 
out the  need  of  any  refining.  Even  in  this  case,  some  slight  adjust- 
ment of  composition  would  probably  be  needed  which  could  be 
effected  in  a  ladle  as  mentioned  in  5. 


CHAPTER  X 
THE  FERRO-ALLOYS  AND  SILICON 

The  production  of  iron  and  steel  in  the  electric  furnace  is  still 
in  its  infancy;  and  will  always  be  limited  by  the  price  of  electrical 
energy;  but  there  are  many  other  uses  to  which  this  source  of  heat 
has  long  been  profitably  applied,  as  has  been  indicated  in  the  first 
two  chapters.  In  some  of  these  processes,  electrical  heat  is  alone 
able  to  produce  the  required  result,  while  in  others  the  value  of  the 
product  and  the  greater  economy  of  the  electrical  method  has  enabled 
it  to  supplant  the  older  processes,  even  though  the  latter  employed 
cheap  fuel  as  the  source  of  heat.  Some  of  these  uses  of  the  electric 
furnace  will  now  be  considered,  and  for  convenience  the  production 
of  the  ferro-alloys  and  silicon  will  be  described  in  this  chapter. 

THE  FERRO-ALLOYS 

The  alloys  of  iron  with  certain  metals,  such  as  manganese,  chro- 
mium, tungsten  and  titanium,  or  with  the  metalloid  silicon,  are  often 
known  as  the  ferros,  and  are  usually  equivalent  to  cast-iron,  that  is 
iron  with  a  large  percentage  of  carbon,  in  which  part  of  the  iron 
has  been  replaced  by  one  of  the  above  metals  or  metalloids.  In 
some  cases,  however,  carbon  is  present  only  in  small  amounts  or 
not  at  all,  and,  on  the  other  hand,  more  than  one  of  the  alloying 
metals  may  be  present  in  the  same  ferro.  The  ferros  are  used  in  the 
production  of  steel  as  convenient  means  for  introducing  into  the 
steel  the  manganese  or  other  metal  which  they  contain;  it  being 
usually  less  costly  to  obtain  these  metals  as  ferro-alloys  than  in  the 
pure  state,  and  the  presence  of  the  iron  is  not  objectionable  in  addi- 
tions made  to  steel;  although  the  carbon,  which  is  also  usually 
present,  is  sometimes  undesirable. 

It  is  well  known  that  many  metals  such  as  manganese  and  silicon, 
which  are  hard  to  reduce,  can  be  more  easily  obtained  in  a  metallic 
state  when  alloyed  with  iron.  Some  recent  researches  in  Dr.  Hut- 
ton's  laboratory1  show  that  these  oxides  are  reduced  to  metals  at  the 
following  temperatures  by  means  of  carbon : 

1  Trans.  Chem.  Soc.  (London)  xcm,  1908,  pp.  1483,  1496,  327,  2101. 
Electrochem  and  Met.  Ind.  VII  1909,  p.  119. 

265 


266  THE  ELECTRIC  FURNACE 

Chromium  oxide  at 1,185°  C.         Silicon  oxide  at 1,460°  C. 

Manganese  oxide  at 1,105°  C.         Zirconium  oxide  at 1,400°  C. 

Uranium  oxide  at 1,490°  C.         Thorium  oxide  at 1,600°  C. 

In  the  presence  of  iron  and  carbon,  some  of  these  metals  can  be 
reduced  at  lower  temperatures,  notably  silicon,  which  is  reduced 
from  its  oxide,  forming  ferro-silicon,  at  about  1,200°  C.  Manganese 
is  reduced  at  about  1,030°  C.  to  form  ferro-manganese,  but  the 
temperature  of  reduction  of  chromium  is  not  lowered  by  the  presence 
of  iron. 

The  ferro-alloys  are  usually  produced  in  an  electric  furnace 
having  a  carbon  hearth,  and  consequently  they  contain  a  certain 
amount  of  this  element.  In  some  cases  furnaces  with  non-car- 
bonaceous hearths  have  been  employed  for  these  alloys  in  order  to 
avoid  their  contamination  with  carbon.  In  other  cases,  the  alloy 
is  made  in  the  usual  way  and  then  is  fused  in  an  electric  furnace 
with  oxides  of  the  metals  which  compose  the  alloy;  this  process 
serving  to  eliminate  most  of  the  carbon  by  reaction  with  the  metallic 
oxide. 

Paul  Girod,1  in  making  electric  steel,  uses  a  number  of  compound 
ferro-alloys,  in  place  of  aluminium,  for  deoxidizing  the  steel  and 
obtaining  sound  ingots;  he  mentions  the  following  alloys: 

(1)  Manganese , 20  per  cent. 

Silicon 20  per  cent. 

Aluminium 12  per  cent. 

(2)  Silicon 40  to  50  per  cent. 

Calcium 20  to  30  per  cent. 

Aluminium •  6  to  10  per  cent. 

(3)  Silicon 40  to  60  per  cent. 

Aluminium 20  to  30  per  cent. 

While  most  of  the  ferro-alloys  are  produced  by  reducing  the  metal- 
lic oxide  with  iron  or  iron-ore  and  carbon  in  an  electric  furnace, 
ferro-vanadium  may  be  produced  by  electrolysis  of  the  oxide  in 
a  fused  electrolyte  of  calcium  and  vanadium  fluorides. 

This  process  has  been  worked  out  by  Gustave  Gin2  who  uses  a 
furnace  lined  with  alumina,  a  carbon  anode  and  a  cathode  of  ferro- 
vanadium*  The  metal  deposits  in  a  spongy  condition  on  the  cathode, 

1  Paul  Girod,  "The  Girod  Electric  Furnace."     Trans.  Am.  Electrochem.  Soc., 
xv,  1909,  p.  138. 

2  Trans.  Am.  Electrochem.  Soc.,  vol.  xv,  1909,  p.  227. 


THE  FERRO-ALLOYS  AND  SILICON  267 

and  when  the  deposit  is  sufficiently  large,  the  furnace  is  emptied 
and  the  deposited  metal  removed.  Gustave  Gin  also  obtains 
vanadium,  as  an  alloy  with  iron  and  silicon,  by  reducing,  in  an 
electric  furnace,  a  mixture  of  the  oxides  of  iron  and  vanadium, 
together  with  silica  and  coke;  or  more  easily,  by  reducing  vanadium 
tri-oxide  by  means  of  rich  ferro-silicon.  For  this  purpose,  60  per 
cent,  ferro-silicon  is  powdered  and  mixed  intimately  with  vanadium 
tri-oxide,  this  is  agglomerated  with  4  per  cent,  to  6  per  cent,  of  coal- 
tar  pitch,  briquetted  and  smelted  in  an  electric  furnace  having  a 
magnesite  hearth. 

The  metal  manganese  resembles  iron  in  many  particulars,  but 
is  more  difficult  to  reduce  from  its  ores.  When  the  reduction  is 
effected  in  the  blast-furnace,  with  iron-ore  to  furnish  enough  iron 
to  collect  and  alloy  with  the  manganese,  some  2.5  or  3  tons  of 
coke  are  required  to  produce  one  ton  of  the  80  per  cent,  ferro-man- 
ganese,  and  about  20  per  cent,  of  the  manganese  is  lost  in  the  slag 
owing  to  the  imperfect  reduction  of  the  ore.  Such  an  operation  is 
very  wasteful,  both  in  fuel  and  in  the  valuable  manganese  ore,  and 
the  electric  furnace  is  so  much  more  economical  in  both  these  par- 
ticulars, that  it  can  be  used  in  competition  with  the  blast-furnace 
method.  Silicon-eisen,  that  is  low-grade  ferro-silicon  containing 
some  10  or  15  per  cent,  of  silicon,  can  be  made  in  the  blast-furnace 
by  using  silicious  charges  and  a  great  excess  of  fuel,  the  silicon  being 
derived  from  the  silica  in  the  charge.  In  the  electric  furnace, 
however,  using  quartz  as  the  source  of  silicon,  with  coke  to  reduce 
the  quartz  to  the  metallic  state,  and  some  iron-ore  or  scrap-iron 
to  alloy  with  the  silicon,  an  alloy  containing  as  much  as  80  per  cent, 
of  silicon  may  be  obtained;  and  the  electric  furnace  ferro-silicon 
has  largely  displaced  the  blast-furnace  product,  as  the  cost  of  the 
former,  per  unit  of  silicon,  is  so  much  less.  Some  other  ferro-alloys 
are  also  made  more  cheaply  in  the  electric  furnace. 

The  ferro-alloys  may  be  produced  in  electric  crucible  furnaces, 
such  as  the  Siemens  vertical-arc  furnace,  Fig.  2>  or  the  Heroult 
ore-smelting  furnace,  Fig.  78,  in  which  a  carbon  electrode  dips 
into  a  carbon-lined  receptacle,  that  forms  the  other  electrode.  In 
such  a  furnace  the  alloy  will  usually  absorb  a  considerable  amount 
of  carbon  from  the  lining,  and  if  a  carbonless  alloy  is  required,  a 
furnace  like  the  Heroult  steel  furnace,  Fig.  93,  should  be  used, 
in  which  two  carbon  electrodes  are  employed,  which  need  not 
touch  the  molten  metal,  and  the  lining  of  the  furnace  is  not  made 
of  carbon. 


268  THE  ELECTRIC  FURNACE 

The  production  and  probable  uses  of  ferro-titanium  are  discussed 
by  Auguste  J.  Rossi,1  who  reduces  titaniferous  iron-ores  in  the  elec- 
tric furnace,  either  with  carbon  or  with  the  assistance  of  molten 
aluminium,  which  serves  to  reduce  the  metal  from  its  ore.  He  has 
obtained  alloys  with  from  10  to  75  per  cent,  of  titanium,  which, 
when  aluminium  was  used  as  the  reducing  reagent,  only  contained 
a  few  tenths  of  i  per  cent,  of  carbon.  Rossi  states  that  titanium  is 
not  really  such  a  bugbear  to  the  iron  metallurgist  as  is  usually  sup- 
posed, but  that  on  the  contrary  ferro-titanium,  added  to  either 
pig-iron  or  steel,  markedly  improves  the  mechanical  properties  of 
the  metal.  In  the  case  of  steel  he  suggests  that  the  well-known 
property  of  titanium  of  combining  with  nitrogen  may  enable  it 
to  remove  this  gas  from  the  molten  metal,  and  in  this  way  to  improve 
its  quality.  Ferro-titanium  is  now  made  in  large  quantities  by 
smelting  titaniferous  iron-ore  in  an  electric  arc-furnace.2  It  is 
used  as  an  addition  to  cast-iron  or  to  steel  mainly  as  a  deoxidizer  and 
cleanser,  being  usually  added  in  the  ladle.  Its  use  has  increased 
very  rapidly  since  the  year  1907  when  it  was  first  employed.  In 
1911  some  400,000  tons  of  steel  were  treated  with  ferro-titanium. 

The  electric  furnace  product  is  rich  in  carbon,  containing  5-8 
per  cent.,  but  a  low-carbon  alloy  can  be  obtained  by  reducing  the 
ore  with  metallic  aluminium,  and  this  contains: 

Titanium 10  to  25  per  cent.         Silicon 0.35  to  i  per  cent. 

Carbon under  i  per  cent.         Aluminium 5  to  8  per  cent. 

The  manufacture  of  ferro-nickel,  ferro-chrome  and  other  alloys 
of  iron  that  are  used  in  the  production  of  steel  is  described  by  O.  J. 
Steinhart.3  Ferro-chrome,  containing  from  50  to  60  per  cent,  of 
chromium,  was  made  at  one  time  by  heating  chromite  with  charcoal 
in  crucibles,  and  later  in  small  blast-furnaces,  but  is  now  made,  al- 
most entirely,  in  the  electric  furnace.  The  Willson  Aluminium  Com- 
pany employed  4,000  E.H.P.,  and  turned  out  200  to  250  tons  per 
month  of  ferro-chrome  having  5  to  6  per  cent,  carbon  and  over  70 
per  cent,  chromium.  Their  works  at  Kanawha  Falls,  W.  Va.,  and 
their  business  and  patents  relating  to  the  manufacture  of  the  ferro- 

1  Rossi,  Mineral  Industry,  vol.  ix,  1901,  p.  715,  and  Trans.  Am.  Inst.  Min. 
Engs.,  vol.  xxxiii,  1903,  p.  191. 

2  Titanium  Alloy  Mfg.  Co.,  1912. 

3  Steinhart,  Trans.  Inst.  Min.  and  Met.,  vol.  xv,  1906,  p.  228. 


THE  FERRO-ALLOYS  AND  SILICON  269 

alloys  have  been  acquired  by  the  Electrical  Metallurgical  Company,1 
who  also  have  works  at  Niagara  Falls.2 

The  Girod  Ferro-Alloy  Works  have  been  described  by  Dr.  R.  S. 
Hutton,3  who-  draws  attention  to  the  wonderful  development  of  the 
hydro-electrical  installations  in  the  French  Alps  and  the  application 
of  this  power  to  electro-metallurgy.  The  three  works  of  the 
"Societe  anonyme  Electrometallurgique,  Precedes  Paul  Girod," 
have  the  following  annual  output: 

5,000  tons  of  50  per  cent,  ferro-silicon. 
1,000  tons  of  30  per  cent,  ferro-silicon. 
2,000  tons  of  ferro-chromium. 
800  to  900  tons  of  ferro-tungsten. 
About  50  tons  of  ferro-molybdenum. 
5  to  10  tons  of  ferro- vanadium. 

The  value  of  the  alloys  sold  is  more  than  $1,800,000  per  annum. 
Two  grades  of  ferro-tungsten  are  produced,  "The  one  containing 
about  85  per  cent,  tungsten,  and  a  maximum  of  0.5  per  cent,  carbon, 
is  chiefly  employed  in  the  manufacture  of  crucible  tool-steels.  The 
other  quality  containing  60  per  cent,  to  70  per  cent,  tungsten,  and 
2  per  cent,  to  3  per  cent,  carbon  is  largely  used  for  the  manufacture  by 
the  open-hearth  process  of  steels  containing  less  than  2.5  per  cent, 
tungsten,  which  are  used  for  the  manufacture  of  springs,  etc." 
Analyses  of  typical  products  of  these  works  are  contained  in  Table 
XX. 

In  dealing  with  these  and  other  products  of  the  electric  furnace, 
it  should  be  remembered  that  they  will  sometimes  evolve  explosive 
gases  if  allowed  to  come  in  contact  with  water.  This  may  be  due 
in  some  cases  to  small  quantities  of  calcium  carbide  formed  at  the 
high  temperature  of  the  electric  furnace,  but  in  one  case,  that  of 
some  ferro-silicon,  which  produced  a  number  of  explosions  in  Liver- 
pool a  few  years  ago,4  the  explosive  gas  was  found  to  be  phosphor- 
etted  hydrogen.  The  alloy  was  very  pure,  containing  nearly  60 
per  cent,  of  silicon,  with  2.7  per  cent,  of  aluminium,  0.2  per  cent,  of 
carbon,  0.14  per  cent,  of  calcium,  0.17  per  cent,  of  magnesium,  and 
0.56  per  cent,  of  phosphorus. 

Manganese,  nickel,  chromium,  tungsten  and  other  metals  can 

1  Electrochemical  Industry,  vol.  v,  p.  248. 

2  Electrochemical  Industry,  vol.  v,  p.  69. 

3  R.  S.  Hutton,  Electrochemical  Industry,  vol.  v,  p.  9. 

4  A.  Dupre  and  M.  B.  Lloyd,  Jour.  Iron  and  Steel  Inst.,  1904,  No.  i,  p.  30. 


270 


THE  ELECTRIC  FURNACE 


TABLE  XX.— ANALYSES  OF  FERRO-ALLOYS 


Ferro- 
manganese 
(blast-furnace) 

Spiegel- 
eisen 
(blast-furnace) 

Silicon- 
spiegel 
(blast- 
furnace) 

Per 

Per 

Per 

Per 

Per 

cent. 

cent. 

cent. 

cent. 

cent. 

Manganese  

82  oo1 

80  oo2 

2O    4O1 

15  oo2 

Iron  

9.90 

12.03 

73.20 

79-93 

66.17 

Carbon 

6.58 

6.80 

5.00 

4-30 

1.65 

Silicon 

I.OO 

0.90 

I.  IO 

0.50 

13.00 

Sulphur  

Trace 

o  02 

Trace 

o  02 

o  08 

Phosphorus 

0.12 

0.25 

0.06 

0.25 

O.  IO 

Arsenic  

o.  10 

o.  10 

Ferro-silicon 


Blast-furnace  Electric-furnace 


Silicon.  .  . 

Per  cent, 
id  8«c3 

Per  cent. 

78  8o4 

Per  cent. 

CQ        AQ& 

Per  cent. 

Iron 

82    Q< 

1  2    64 

36  8c 

6L  •  Vu 

Manganese  
Aluminium.  .  . 

o-34 

0.30 
A    76 

0.08 

2    73 

3-92 

Calcium  

2    32 

O    14 

O    7O 

Magnesium  v 

O    22 

O    17 

o  26 

Carbon  

i  66 

O    'J  ^ 

o  218 

Sulphur.   . 

o  08 

o  008 

Trace 

«-  •  5'-' 

Phosphorus 

O    12 

u-  uoo 

Chromium  

o  16 

I    O2 

Copper  

O.O4 

O   OI 

Tungsten  

O.  OO 

0.25 

also  be  obtained  in  a  carbon-free  and  nearly  pure  state,  suitable 
for  use  in  the  manufacture  of  special  varieties  of  steel,  by  the  Gold- 
schmidt  process  of  mixing  the  oxide  of  the  metal  with  powdered 
aluminium  and  igniting  the  charge  by  means  of  a  small  primer, 
which  starts  the  reaction  between  the  oxide  and  the  aluminium. 
The  reaction  once  started  continues  throughout  the  mass,  producing 
an  intense  heat,  which  is  sufficient  to  melt  the  reduced  metal  and  the 
resulting  alumina. 

The  metalloid  silicon,  on  account  of  its  strong  affinity  for  oxygen, 
can  be  used  instead  of  aluminium  for  the  reduction  of  such  metals  as 

1  F.  W.  Harbord,  The  Metallurgy  of  Steel,  p.  53. 

2  P.  Longmuir,  Elementary  Practical  Metallurgy,  Iron  and  Steel,  p.  144. 

3  P.  Longmuir,  Elementary  Practical  Metallurgy,  Iron  and  Steel,  p.  81. 

4  G.  W.  Gray,  Jour.  Iron  and  Steel  Institute,  1901,  No.  2,  p.  144. 

5  G.  W.  Gray,  Jour.  Iron  and  Steel  Institute,  1904,  No.  i,  p.  32. 


THE  FERRO-ALLOYS  AND  SILICON 


271 


Ferro-chromium 


Crucible 
furnace 


Electric  furnace1 


Chromium. .. 

Iron 

Carbon 

Silicon. 

Manganese. . . 
Aluminium. . . 
Magnesium. . 

Sulphur 

Phosphorus . . 


Per 

cent. 

45-o 

4S-o 

8.6 

c.6 

0.4 

o.o 

o.o 

0.05 

0.05 


Per 

cent. 

60.00 

30.00 

9.1 

c.S 

o-3 

o.o 

o.o 

0.05 
0.05 


Per 

cent. 

67.20 

3i-35 
0.90 
o.  19 

0.12 
O.OO 

o.  19 

0.006 

O.C2I 


Per 

cent. 

64.17 

32.47 

2-34 

0.38 

O.  21 

0.13 
0.23 
0.023 
O.O2 


Per 
cent. 
67.05 
27.05 

4-25 
0.60 
0.46 

0.  22 
0.31 
O.O2 


Per 

cent. 

65.90 

23-44 

8.58 

1.26 

0.44 

0.18 

0.14 

O.O2 
0.02 


Ferro-tungsten 
(Electric  furnace)1 


Ferro-vanadium 
(Electric  furnace)1 


Per  cent. 


Per  cent. 


Per  cent,  j  Per  cent. 


Tungsten 85.15          71.80       Vanadium 52.80          34.10 

Iron 14.12          24.35       Iron 45-84          64.22 

Carbon 0.45            2.58       Carbon 1.04            1.42 

Silicon 0.13            0.36       Silicon 0.09           0.12 

Manganese 0.085          0.78       Aluminium o.oo           o.oo 

Sulphur 0.021          0.02       Sulphur 0.025          0.03 

Phosphorus 0.018          0.008     Phosphorus 0.02            0.009 

Ferro-molybdenum 

(Electric  furnace)2 

Per  cent.        Per  cent. 

Molybdenum 79 . 15  83 . 80 

Iron 17.52  12.72 

Carbon 3.24  3.27 

Sulphur 0.021  0.02 

Phosphorus 0.028  0.027 

Ferro-titanium 
(Electric  furnace).2 

Per  cent. 

Titanium 15  to  18 

Iron,  about 76 

Carbon 5  to    8 

Silicon 0.35  to    i 

Sulphur Trace 

Phosphorus Trace 

JR.  S.  Hutton,  Electrochemical  Industry,  vol.  v,  p.  10. 
2  Titanium  Alloy  Mfg.  Co.,  1912. 


272  THE  ELECTRIC  FURNACE 

chromium,  tungsten  and  molybdenum  from  their  oxides,  and  for 
obtaining  alloys  of  these  metals  with  iron  or  nickel.  Mr.  F.  M. 
Becket1  has  patented  this  process,  and  describes  the  production  of 
ferro-chrome,  low  in  carbon  and  silicon,  by  feeding  a  mixture  of 
chromite  and  metallic  silicon  into  an  electric  furnace.  The  oxides 
of  iron  and  chromium,  contained  in  the  chromite,  are  reduced  to 
the  metallic  state  by  reacting  with  the  silicon  according  to  the 
following  equations: 


i  =  2Fe+SiO2. 

The  silica  resulting  from  the  reaction  is  slagged  off  by  the  basic 
impurities  present  in  the  chromite.  An  excess  of  the  chromite 
is  used  to  prevent  any  of  the  silicon  remaining  unoxidized  and 
alloying  with  the  ferro-chrome. 

Mr.  Becket2  also  patents  the  use  of  silicon  or  ferro-silicon  for 
reducing  metals,  particularly  the  metals  molybdenum  and  vana- 
dium, from  their  sulphide  ores.  The  following  equation  shows  the 
action  when  silicon  and  molybdenite  are  used: 

MoS2+Si  =  Mo+SiS2. 

Mr.  E.  F.  Price3  has  also  secured  a  patent  for  the  production  of 
low-carbon  ferro-chromium,  etc.,  by  the  use  of  ferro-silicon.  He 
obtains  ferro-silicon  in  an  electric  furnace  and  then  taps  it  into  a 
second  electric  furnace,  where  it  is  made  to  react  with  the  chromite, 
for  the  production  of  ferro-chrome. 

A  paper  by  R.  M.  Keeney,4  "Electric  Smelting  of  Chromium, 
Tungsten,  Molybdenum  and  Vanadium  ores,"  which  has  just 
appeared,  gives  a  good  account  of  these  processes. 

Ferro-silicon5  is  made  by  smelting,  in  an  electric  furnace,  a  mix- 
ture of  silica,  carbon  and  iron  or  iron-ore.  Originally  iron-ore  was 
generally  employed  but  now  it  is  more  usual  to  employ  turnings 
of  steel  or  wrought-  iron.  Silica  is  supplied  in  the  form  of  quartz 
or  quartzite,  instead  of  sand  which  would  be  liable  to  choke  the 

1  F.  M.  Becket,  U.  S.  patent  854,018,  Electrochemical  Industry,  vol.  v,  p.  237. 

2  F.  M.  Becket,  U.  S.  patent  855,157,  Electrochemical  Industry,  vol.  v,  p.  237. 
3E.  F.  Price,  U.  S.  patent,  852,347,  Electrochemical  Industry,  vol.  v,  p.  278. 
4R.  M.  Keeney,  Am.  Electrochem.  Soc.,  xxiv,  1913. 

5  British  Government  Report  on  Manufacture,  Uses  and  Transport  of  Ferro- 
silicon,  by  S.  M.  Copeman,  S.  R.  Bennett  and  H.  W.  Hake,  Met.  and  Chem. 
Eng.,  viii,  1910,  p.  133  (also  see  p.  115). 


THE  FERRO-ALLOYS  AND  SILICON 


273 


furnace.  Anthracite  coal  is  generally  used  as  the  source  of  carbon, 
but  sometimes  gas  coke  is  employed.  Anthracite  is  preferable  to 
coke  because  the  latter  is  so  good  an  electrical  conductor  that  it  is 
not  easy  when  using  it  to  obtain  a  sufficiently  high  electrical  resist- 
ance in  the  furnace. 

The  materials  employed  should  be  as  pure  as  possible,  and  in 
particular  should  be  free  from  phosphorus  and  arsenic,  as  ferro- 
silicon  containing  these  elements  is  apt  to  form  poisonous  and  explo- 
sive gases. 


^ 

FIG.  112. — Ferro-silicon  furnace. 

A  furnace  used  for  making  ferro-silicon  is  shown  in  Fig.  112. 
It  is  usually  circular  in  plan,  built  of  fire-bricks,  B,  in  an  iron  or  steel 
casing,  and  often  having  a  lining  of  carbon.  The  upper  electrode 
is  of  carbon,  square  in  section,  and  having  a  water-cooled  electrode- 
holder,  and  an  iron  casing,  E,  to  prevent  wasting  and  oxidation. 
The  lower  electrode,  C,  passes  through  and  fills  the  bottom  of  the 
furnace.  It  may  be  formed  in  situ  by  ramming  retort-carbon  and 
tar.  The  voltage  employed  varies  from  about  40  to  75  volts  and 

18 


274  THE  ELECTRIC  FURNACE 

the  current  from  10,000  to  15,000  amperes;  the  power  used  varying 
from  250  or  300  h.p.  up  to  750  or  800  h.p. 

At  the  Keller  Leleux  works  the  Keller  furnace  is  employed  (see 
Fig.  79).  This  furnace  is  equivalent  to  two  furnaces  built  together 
and  coupled  electrically  in  series,  thus  dispensing  with  the  lower 
electrodes. 

In  making  up  the  charge  for  the  furnace,  silica  and  carbon  are 
added  in  the  proportion  given  by  this  equation: 

.     Si02+2C  =Si+2CO 
60  +24  =28+  56 

Iron  is  added  in  amount  depending  on  the  percentage  of  silicon  that 
is  desired.  The  higher  the  grade  of  ferro-silicon,  the  smaller  would 
be  the  amount  of  iron  to  be  added.  Allowance  must,  of  course,  be 
made  for  the  impurities  in  the  quartz,  anthracite  and  iron  employed. 
Thus  in  making  a  50  per  cent,  ferro-silicon  from  quartzite  containing 
96  per  cent,  silica,  and  anthracite  of  82  per  cent,  carbon,  10  per  cent, 
ash  and  8  per  cent,  water  and  gases,  the  following  charge  would  be 
used: 

Quartzite  150  kg.  =  144  kg.  SiC>2  or  66  kg.  silicon. 

Anthracite  72  kg.  =   59  kg.  carbon  and  7  kg.  of  silica, 

alumina,  etc. 

Steel  turnings     55  kg.  =   54  kg.  iron. 
277  kg. 

This  charge  should  yield  about  66  kg.  of  silicon  and  54  kg.  of 
iron,  but  as  the  loss  of  silicon  through  imperfect  reduction  and  by 
volatilization  will  be  greater  than  the  loss  of  iron,  the  resulting  alloy 
will  contain  about  50  per  cent,  of  silicon. 

The  quartz  and  anthracite  are  crushed  to  nut  size,  mixed  with  the 
steel  turnings  in  correct  proportions  and  shovelled  into  the  furnace. 
The  furnace  operates  continuously  and  the  ferro-silicon  is  tapped 
into  sand  moulds  at  intervals ;  the  slag  being  skimmed  off  the  molten 
metal  in  the  mold  by  a  log  of  wood. 

The  production  of  ferro-silicon  is  described  by  Albert  Keller,1 
who  states  that  at  Livet,  with  4,000  h.p.,  he  was  able  to  turn  out 
20  tons  of  30  per  cent,  ferro-silicon  per  day,  and  that  i  ton  of  the 
alloy  requires  3,500  kw. -hours  for  its  production  from  quartz,  scrap- 
iron  and  coke,  the  furnaces  being  each  of  650  h.p. 

1  Keller,  Jour.  Ison  and  Steel  Inst.,  1903,  vol.  i,  p.  166. 


THE  FERRO-ALLOYS  AND  SILICON  275 

The  British  Coalite  Co.  at  their  Wednesfield  experimental  station,1 
made  ferro-silicon  from  a  sandstone  rock,  precipitated  ferric  oxide, 
pitch  and  10  per  cent,  of  anthracite.  The  crushed  sandstone,  iron 
oxide  and  anthracite  were  mixed  together  with  the  pitch  and  heated 
to  drive  off  the  volatile  hydrocarbons.  The  product  was  broken 
into  small  lumps  and  smelted  in  the  furnace.  This  use  of  pitch 
ensures  an  intimate  mixture  of  carbon  with  the  silica  and  iron  oxide, 
and  the  mass  has  a  suitable  electrical  resistance  for  use  in  the 
furnace.  The  ferric  oxide  and  pitch  were  both  by-products  from 
other  processes. 

Ferro-silicon  is  usually  made  in  an  electric  furnace  like  that  shown 
in  Fig.  112,  in  which  the  hearth  of  the  furnace  is  made  of  carbon  and 
forms  an  electrode.  Even  in  the  presence  of  an  abundance  of  carbon, 
ferro-silicon  does  not  take  up  a  large  percentage  of  this  element. 
A  sample  of  ferro-silicon  produced  in  a  blast-furnace  only  contained 
1.6  per  cent,  of  carbon;  silicon  having  the  effect  of  preventing  carbon 
from  combining  with  iron.  Ferro-silicon  produced  in  the  electric 
furnace  contains  only  a  few  tenths  per  cent,  of  carbon  as  shown  in 
Table  XX. 

Fig.  113  represents  a  ferro-silicon  furnace  for  4,000  to  6,000  kw.,  in 
which  three  electrodes  are  provided  for  a  three-phase  supply.  The 
furnace  is  lined  with  carbon,  but  the  lining  does  not  form  one  elec- 
trode (as  in  Fig.  112).  On  account  of  the  difficulty  of  drilling  through 
the  solidified  slag  with  a  steel  bar,  in  order  to  tap  the  ferro-silicon, 
a  tapping  electrode  is  used,  mounted  on  a  carriage,  A,  and  supplied 
with  current  through  the  cable,  K.  The  figure  shows  how  the  trans- 
formers, T,  can  be  protected  from  the  heat  and  dust,  and  yet  can  be 
placed  near  to  the  furnace  so  that  a  moderate  length  of  cables  will 
serve  to  connect  them  to  the  electrode  holders.  V  is  a  fan  for  cooling 
the  transformers,  and  W  is  a  motor  for  raising  the  electrodes. 

The  Helfenstein  three-phase  furnace2  consists  of  three  or  more 
shafts,  placed  side  by  side,  each  having  its  own  vertical  top  elec- 
trode, but  all  being  connected  together  electrically  by  a  common 
conducting  bottom  which  forms  the  bottom  electrode  of  each  shaft. 
The  furnace  top  is  closed,  having  provision  for  feeding  the  charge 
and  adjusting  the  electrodes  without  allowing  the  gases  to  escape. 
These  furnaces  are  made  to  take  12,000  H.P.  in  three  shafts. 

A  recent  invention  of  Mr.  Tone3  relates  to  the  fusion  in  the  elec- 

1  Met.  and  Chem.  Eng.,  viii,  1910,  p.  134. 
,2  Met.  and  Chem.  Eng.,  x,  1912,  p.  686. 
3  Electrochem.  and  Met.  Ind.,  vii,  1909,  pp.  35  and  192. 


276 


THE  ELECTRIC  FURNACE 


THE  FERRO-ALLOYS  AND  SILICON  277 

trie  furnace  of  clay  with  iron  and  coke;  the  amount  of  coke  being  just 
sufficient  for  the  reduction  of  the  silicon  in  the  clay.  The  silicon  is 
reduced  and  mixes  with  the  iron  in  the  charge,  forming  ferro-silicon. 
The  remainder  of  the  clay  is  alumina,  and  this  melts,  forming  the 
product  alundum,  or,  as  it  is  called  when  produced  by  this  process, 
Aloxite. 

Many  serious  accidents  have  occurred  in  connection  with  the 
storage  and  transportation  of  ferro-silicon.  This  substance  is 
liable  to  give  off  poisonous  and  inflammable  gases;  people  have 
been  killed  by  breathing  these  gases,  and  explosions  have  taken 
place,  due  to  their  spontaneous  ignition. 

The  gas  evolved  is  mainly  phosphoretted  hydrogen,  due  to  the 
action  of  damp  air  on  calcium  phosphide  in  the  alloy,  and  to  a  less 
extent  arseniuretted  hydrogen.  These  gases  are  very  poisonous 
and  ignite  spontaneously  or  very  easily;  for  example  by  means  of 
a  spark  caused  by  friction  between  the  pieces  of  metal.  In  order 
to  diminish  this  danger  the  materials  used  in  the  manufacture  of 
ferro-silicon  should  be  as  free  as  possible  from  phosphorus  and  arsenic. 
Thus,  for  example,  iron  or  steel  turnings  should  be  used  instead  of 
cast-iron  turnings,  as  the  latter  contain  a  larger  amount  of 
phosphorus. 

It  has  been  found  that  certain  grades  of  ferro-silicon  are  liable 
to  spontaneous  disintegration,  and  some  even  crumble  to  powder 
after  a  few  weeks  or  months;  this  disintegration  being  usually  accom- 
panied by  the  evolution  of  evil-smelling  and  poisonous  gases.  The 
grades  that  are  at  all  liable  to  this  are  from  30  per  cent,  to  40  per 
cent,  and  from  47  per  cent,  to  62  per  cent,  of  silicon.  It  has  there- 
fore been  recommended  that  ferro-silicon  should  be  made  with  sili- 
con contents  above  70  per  cent,  or  below  30  per  cent.,  as  it  is  found 
that  these  grades  are  perfectly  safe  to  store  and  handle. 

In  handling  any  other  grades,  it  is  desirable  to  break  up  the 
ferro-silicon  and  to  store  it  in  a  dry,  well-ventilated  place  for  a 
month  before  shipment.  In  shipping  by  sea  it  should  be  placed 
on  deck  if  possible,  failing  which  it  must  be  in  a  well- ventilated  place, 
separated  from  inhabited  parts  of  the  vessel. 

SILICON 

It  has  been  estimated  by  Dr.  F.  W.  Clarke1  that  silicon  forms 
27.4  per  cent,  of  the  contents  of  the  solid  crust  of  the  earth.  It 

1Dr.  F.  W.  Clarke,  of  the  United  States  Geological  Survey.  Quoted  in 
Electrochemical  Industry,  vol.  iii,  p.  409. 


278 


THE  ELECTRIC  FURNACE 


exists  in  combination  with  oxygen  as  silica,  which  constitutes,  accord- 
ing to  this  estimate,  58.3  per  cent,  of  the  earth's  crust.  Although 
so  widely  distributed,  silicon  has  so  strong  an  affinity  for  oxygen 
that  until  recently  it  could  only  be  obtained  in  small  amounts. 

Alloyed  with  certain  metals,  silicon  has  long  been  of  metallurgical 
importance.  Ferro-silicon,  already  referred  to,  has  been  employed 
in  the  manufacture  of  steel  as  a  deoxidizer  and  to  prevent  the  forma- 
tion of  blow-holes  in  steel  ingots.  Cast-iron  contains  a  small 
amount  of  silicon,  which  has  a  very  great  effect  on  the  properties 
of  the  iron. 

Silicon  is  formed  by  the  action  of  carbon  on  silica  in  the  elec- 
tric furnace  at  a  temperature  which  has  been  stated  to  be  1460°  C.; 


FIG.  114. — Tone's  arc  furnace  for  silicon. 

but  a  higher  temperature  is  probably  essential  for  a  rapid  reduction 
of  this  metalloid.  It  is  not  easy  to  obtain  silicon  in  this  manner 
because  it  is  volatile  at  the  temperature  of  its  formation,  and  because 
it  easily  forms  a  carbide,  so  that  unless  great  care  is  taken,  the  silicon 
is  largely  lost  by  volatilization  and  what  is  obtained  is  contaminated 
by  admixture  with  the  carbide  SiC. 

The  reduction  of  silica  to  silicon  has  been  carried  out  successfully 
by  Mr.  F.  J.  Tone.1  His  first  furnace,  which  is  shown  in  Fig.  17, 
was  a  resistance  furnace.  He  used  this  form  of  furnace  because  the 

1  F.  J.  Tone,  Production  of  Silicon  in  the  Electric  Furnace.  Trans.  Amer. 
Electrochem.  Soc.,  vol.  vii,  1905,  p.  243;  Production  of  Silicides  and  Silicon 
Alloys,  U.  S.  patent  842,273;  See  Electrochemical  Industry,  vol.  v,  p.  141. 


THE  FERRO-ALLOYS  AND  SILICON  279 

temperature  could  be  regulated  better  in  it  than  in  an  arc- furnace,  and 
the  volatilization  of  the  silicon  and  the  formation  of  carbide  of  silicon 
would  therefore  be  less.  More  recently  Mr.  Tone  has  employed 
arc-furnaces  which  are  designed  to  permit  the  collection  of  the  sili- 
con in  the  bottom  of  the  furnace;  this  being  at  a  far  lower  temper- 
ature than  that  of  the  arc,  but  still  above  the  melting  temperature 
of  silicon.  One  form  of  this  furnace  is  shown  in  Fig.  114.  There 
are  four  vertical  carbon  electrodes;  electrical  connections  being 
made  to  the  upper  electrodes,  and  the  lower  electrodes  being  con- 
nected together  through  the  carbon  lining  of  the  furnace.  In  this 
way  two  arcs  are  produced  in  the  middle  of  the  charge,  which  is 
a  mixture  of  quartz  and  coke.  The  silicon  volatilizes  as  soon  as 
it  is  formed,  and  condenses  in  the  porous  mass.  It  then  filters 
down  through  the  charge  to  the  bottom  of  the  furnace,  from  which 
it  is  tapped  at  intervals. 

A  modified  form  of  this  furnace,1  has  a  horizontal  carbon  grid 
placed  across  the  furnace,  below  the  level  of  the  arcs,  so  as  to  support 
the  charge  and  maintain  a  clear  space  for  the  collection  of  the  silicon 
in  the  bottom  of  the  furnace. 

A  furnace,  designed  by  Seaward  Kouglegan,2  consists  of  a  fire-brick 
chamber  having  a  carbon  hearth  and  two  depending  electrodes. 
The  silicon  collects  on  the  hearth  and  serves  to  carry  the  electric 
current  from  one  electrode  to  the  other.  The  electrodes  have  each 
a  cross-section  of  4  sq.  ft.  for  a  furnace  using  15,000  amperes  at 
30  volts.  The  charge  consists  of  flint  rock,  broken  to  the  size  of 
i  in.,  and  coke  crushed  to  pass  through  a  lo-mesh  sieve.  In  start- 
ing the  furnace,  75  parts  of  flint  rock  are  used  with  25  parts  of  coke, 
this  proportion  furnishing  an  excess  of  silica.  Later  70  parts  of 
flint  rock  are  used  with  30  of  coke,  which  indicate  a  small  excess 
of  silica  over  the  theoretical  proportion  of  5  parts  of  silica  to  2  parts 
of  carbon.  The  patent  specification  indicates  an  arc  between  each 
electrode  and  the  molten  silicon,  but  as  this  would  cause  the  silicon 
to  volatilize,  it  is  probable  that  the  arcs  are  not  actually  in  contact 
with  the  molten  product. 

The  furnaces  in  use  at  the  Acheson  Carborundum  Works3  have 
two  depending  electrodes,  which  extend  to  a  considerable  depth 
into  the  charge  of  coke  and  sand.  The  furnace  is  built  of  fire-brick 

1  F.  J.  Tone,  U.  S.  patent  937,120,  Oct.  18,  1909,  and  Electrochem.  and  Met, 
Jnd.,  vii,  1909,  p.  495. 

2  Seaward  Kouglegan,  Electrochem.  and  Met.  Ind.,  vol.  vii,  1909,  p.  223. 

3  Electrochem.  and  Met.  Ind.,  vii,  1909,  p.  192. 


280  THE  ELECTRIC  FURNACE 

and  is  lined  with  carbon.  Each  furnace  uses  1,200  h.p.  and  the 
silicon  is  tapped  every  few  hours  in  ingots  of  600  to  800  Ib. 

Apparently  the  present  type  of  silicon  furnace  is  a  simple  chamber 
constructed  of  brickwork  and  lined  with  carbon.  It  has  two  or 
three  depending  electrodes  and  the  current  passes  between  these 
through  the  white-hot  charge  and  the  molten  silicon  which  collects 
in  the  bottom  of  the  furnace.  The  electrodes  will  be  raised  so 
high  in  the  furnace  that  the  silicon  will  not  be  overheated,  and  a 
thick  covering  of  charge  will  be  maintained  to  act  as  a  filter  and 
condenser  to  prevent  losses  by  the  escape  of  silicon  vapor  with  the 
carbon  monoxide. 

H.  N.  Potter1  has  patented  a  process  for  the  production  of  silicon 
from  a  mixture  of  silica  and  silicon  carbide  according  to  the  equation: 


SiO2  +2  SiC  =  3Si  +2CO. 

The  silicon  produced  is  liable  to  be  contaminated  with  silicon  carbide, 
but  this  is  eliminated  by  reaction  with  a  bed  of  silica  on  which  the 
molten  silicon  collects. 

Silicon  produced  in  the  electric  furnace2  is  a  brittle  crystalline  body 
with  a  dark  silver  luster.  Its  specific  gravity  is  2.34  and  it  melts  at 
1,430°  C.  It  is  not  pure,  however,  but  contains  about  i  per  cent,  of 
iron,  i  .  5  per  cent,  of  aluminium,  and  2  per  cent  of  carbon. 

The  heat  of  oxidation  of  silicon  has  been  determined  by  Dr.  H.  N. 
Potter,3  who  gives  215,  692  calories  as  the  heat  formation  of  i  grm. 
molecule  of  silica.  Using  this  figure  it  can  be  shown  that  the  oxida- 
tion of  silicon  affords  more  heat  per  unit  weight  of  oxygen  than 
the  oxidation  of  any  of  the  metals  except  aluminium  and  the 
alkaline  earth  metals  such  as  magnesium  and  calcium.  The  great 
affinity  of  silicon  for  oxygen  has  enabled  it  to  be  used  for  the 
reduction  of  metals  such  as  chromium  and  tungsten  in  the  electric 
furnace.4 

Silicon  is  made  commercially  in  grades  containing  from  90  to  97 
per  cent,  of  silicon,  as  shown  by  the  following  analyses: 

1  H.  N.  Potter,  U.  S.  patent  908,130,  Dec.  29,  1908,  and  Electrochem.  and 
Met.  Ind.,  vii,  1909,  p.  86. 

2  F.  J.  Tone,  Production  of  Silicon  in  the  Electric  Furnace.     Trans.  Am. 
Electrochem.  Soc.,  vii,  1905,  p.  243. 

3H.  N.  Potter,  Trans.  Amer.  Electrochem.  Soc.,  vol.  xi,  abstracted  in  Electro- 
chemical Industry,  vol.  v.,  p.  229, 

4  F.  M.  Becket,  patents  described  in  Electrochemical  Industry,  vol.  v,  p.  237. 
Trans.  Amer.  Electrochem.  Soc.,  vol.  vii,  p.  249. 


THE    FERRO-ALLOYS  AND  SILICON  281 

Per  cent.  Per  cent. 

Silicon 90.60  95-71 

Iron 6 .  70  2 . 24 

Manganese o .  08  

Aluminium 2.35  i .  96 

Phosphorus 0.02  o.oi 

Carbon 0.22  0.08 

Sulphur o.oo  o.oo 

The  go-per  cent,  grade  is  largely  used  in  steel-making,  replacing 
the  higher  grades  of  ferro-silicon.  Silicon  is  also  employed,  instead 
of  aluminium,  as  a  reducing  agent  in  the  manufacture  of  low-carbon 
ferro-alloys  by  the  "thermit"  process. 

Silicon  is  produced  commercially  in  large  quantities  by  the  Car- 
borundum Company  at  Niagara  Falls.1  It  is  of  great  value  on 
account  of  its  resistance  to  acids  and  it  can  now  be  cast  in  the  various 
shapes  needed  in  the  chemical  industry.  Cast  silicon  has  a  density 
of  2.5  to  2.6,  an  electrical  resistivity  of  0.15  ohms  per  centimeter 
cube  at  18°  C.,  decreasing  rapidly  with  rise  of  temperature;  it  melts  at 
1,430°  C.,  and  its  boiling-point  has  been  determined  (by  calculation) 
to  be  2,800°  C. 

Silicon-copper,  an  alloy  of  silicon,  is  used  as  a  deoxidizer  in  making 
castings  of  copper  and  copper  alloys,  in  the  same  way  that  the  ferro- 
alloy of  silicon  is  used  in  making  steel  castings.  Silicon-copper  is 
made  in  the  electric  furnace  by  the  Cowles  Electric  Smelting  and 
Aluminium  Company,  at  Lockport,  N.  Y.2 

1  Electrochem.  and  Met.  Ind.,  vii,  1909,  p.  192. 

The  Carborundum  Company  Catalogue  of  Cast  Silicon  Chemical  Ware. 

2  Silicon-copper  in  the  Brass  Foundry,  Electrochemical  Industry,  ii,  1904,  p. 


CHAPTER  XI 
GRAPHITE  AND  CARBIDES 

The  production  in  the  electric  furnace  of  graphite  and  of  several 
carbides,  particularly  carborundum  and  calcium  carbide,  will  be 
considered  in  this  chapter. 

GRAPHITE 

The  elementary  substance  carbon  exists  in  nature  in  three  distinct 
forms:  Amorphous  carbon,  graphite  and  the  diamond.  Amorphous 
carbon  exists  in  a  nearly  pure  state  in  such  substances  as  charcoal, 
lampblack,  petroleum-coke,  and  the  ordinary  electric-light  carbons. 
Graphite  derives  its  name  from  its  property  of  marking  paper,  and  is 
largely  used  for  this  purpose  in  the  common  "lead  pencil."  Plum- 
bago and  black-lead  are  other  names  for  graphite,  which  date  from  a 
time  before  the  true  nature  of  graphite  had  been  discovered,  and  when 
it  was  supposed  to  be  closely  related  to  lead  and  certain  of  its  ores.1 
Natural  graphite  is  classified  as  crystalline  and  amorphous;  the  former 
occurs  in  flakes  or  flaky  masses  and  can  easily  be  freed  from 
associated  earthy  matter,  while  the  latter,  which  must  not  be  confused 
with  amorphous  carbon,  mentioned  above,  does  not  occur  in  flakes, 
and  is  therefore  not  so  easily  separated  from  the  clayey  and  other 
impurities  with  which  it  is  frequently  intimately  associated.2  Crys- 
talline graphite  is  largely  used  in  the  manufacture  of  pencils,  cru- 
cibles and  lubricants,  while  the  less  valuable  amorphous  graphite  is 
utilized  for  paints  and  foundry  facings.  The  graphite  from  the  Bor- 
rowdale  mines  in  Cumberland,  although  amorphous,  was  famous  for 
many  years  as  the  best  for  making  pencils;  being  of  great  purity. 

The  diamond,  the  remaining  form  of  carbon,  is  remarkable  for 
its  great  hardness,  its  crystalline  form,  and  its  transparency  when 
pure.  It  has  been  produced  artificially  by  crystallization  under 
pressure  from  a  solution  of  carbon  in  molten  iron,  but  the  process  has 
not  attained  any  commercial  success.3 

1  Graphite:  its  formation  and  manufacture,  by  E.  G.  Acheson,  Jour.  Franklin 
Institute,  1899. 

2  Graphite,  by  E.  K.  Judd,  Mineral  Industry,  vol.  xiv,  p.  309. 

8  Artificial  diamonds,  Electrochemical  Industry,  vol.  iv,  p.  343. 

282 


GRAPHITE  AND  CARBIDES  283 

Graphite  differs  from  amorphous  carbon  in  the  following  partic- 
ulars :  It  has  a  somewhat  higher  specific  gravity,  it  is  a  better  electrical 
conductor,  and  is  less  easily  oxidized  by  air  at  a  red-heat  or  by  cer- 
tain chemical  reagents.  Its  greater  resistance  to  oxidation  enables 
it  to  be  used  in  the  manufacture  of  crucibles,  and  this  and  its  good 
electrical  conductivity  render  it  valuable  as  a  material  for  electrodes 
for  various  electro-chemical  and  electric-furnace  operations. 

It  has  been  known  for  a  long  time  that  amorphous  carbon  and  the 
diamond  could  be  converted  into  graphite  by  exposure  to  very  high 
temperatures  and  in  other  ways.  The  conversion  of  amorphous 
carbon  into  graphite  by  the  action  of  heat  is  only  accomplished  at 
the  highest  temperatures  of  the  electric  furnace,  and  even  then  not 
readily.1  When,  however,  some  metal  like  iron  or  nickel,  which 
has  the  property  of  dissolving  carbon  when  in  the  molten  state, 
is  saturated  with  that  substance,  and  then  allowed  to  cool  slowly, 
the  carbon  will  crystallize  or  separate  from  the  cooling  metal  as 
flakes  of  graphite.  The  separation  of  graphite  from  molten  pig-iron 
can  be  noticed  very  easily  in  a  blast-furnace  casting  house.  The 
method  by  which  large  amounts  of  graphite  are  now  artificially 
produced  depends  on  the  formation  of  carbides  of  iron,  silicon,  etc., 
and  the  subsequent  decomposition  of  these  carbides  at  a  still  higher 
temperature;  the  iron,  etc.,- being  driven  off  in  the  state  of  vapor, 
leaving  the  carbon  in  the  form  of  graphite  and  of  a  high  degree  of 
purity. 

The  decomposition  of  carbide  of  silicon  yielding  graphite  in  the 
hottest  part  of  the  carborundum  furnace  had  been  noticed  by  Dr. 
Acheson  who  investigated  the  matter  and  found  that  pure  forms  of 
carbon  were  only  slightly  changed  into  graphite  in  the  electric  fur- 
nace, but  that  impure  carbon  such  as  ordinary  coke,  or  carbon  to 
which  certain  substances  such  as  iron  oxide,  silica,  or  alumina  had 
been  added,  were  largely  converted  into  graphite.  Dr.  Acheson 
patented  the  electric-furnace  production  of  graphite  in  i8962  and 

1  F.  J.  FitzGerald,  The  Conversion  of  amorphous  carbon  to  graphite,  Jour. 
Franklin  Institute,  1902. 

W.  C.  Arsem,  "Transformation  of  Other  Forms  of  Carbon  into  Graphite." 
Trans.  Am.  Electrochem.  Soc.,  xx,  1911,  p.  105. 

2  E.  G.  Acheson,  U.  S.  patent  568,323,  Sept.  29,  1896.     Converts  carbonaceous 
materials  such  as  mineral  coal,  coke,  charcoal,  gas-carbon  and  carbides  into 
practically  pure  graphite,  by  employing  a  material  containing  a  considerable 
proportion  of  mineral  matter,  or  mixing  it  with  an  oxide  or  oxides,  such  as  silica, 
clay,  alumina,  magnesia,  lime  or  iron  oxide,  and  heating  the  mixture  in  an  electric 
furnace,  Electrochemical  Industry,  vol.  iii,  p.  482. 


284  THE  ELECTRIC  FURNACE 

its  commercial  development  has  been  so  rapid  that  in  1905 
the  production  of  artificial  graphite  was  greater  than  the  whole  out- 
put of  natural  crystalline  graphite  in  the  United  States.  During 
the  years  1907-09  the  annual  output  of  Acheson  graphite  was 
7,000,000  Ib. 

The  electric-furnace  production  of  graphite1  is  illustrated  in 
Figs.  115  and  116;  the  former  showing  a  furnace  for  the  conversion 
of  anthracite  into  bulk  graphite,  while  the  latter  illustrates  the 
graphitization  of  electrodes  or  other  articles  of  amorphous 
carbon. 

Anthracite  has  been  selected  as  the  most  suitable  material  for  the 
production  of  graphite  in  bulk ;  the  impurities  which  are  disseminated 
through  it  serving  as  carbide-forming  materials  which  render  possible 
its  conversion  into  graphite.  The  graphite  furnace  consists,  as  is 
shown  in  Fig.  115,  of  a  long  trough  which  contains  the  anthracite, 
and  of  two  electrodes  which  are  situated  at  the  ends  of  the  furnace. 
As  the  cold  anthracite  is  a  very  poor  conductor  of  electricity,  a  core, 
C,  of  carbon  rods  is  needed  to  carry  the  current,  until  the  charge 
becomes  heated.  The  furnace  consists  of  a  permanent  base,  B,  and 
end  walls,  AA,  which  support  the  electrodes.  The  side  walls,  DD, 
are  not  permanent  but  can  be  pulled  down  after  a  run.  The  base 
of  the  furnace  is  shown  supported  on  bricks  so  as  to  allow  of  air- 
cooling,  but  this  precaution  is  not  always  taken.  The  electrodes 
are  made  of  a  number  of  graphite  rods,  £,  which  are  set  in  a  block  of 
carbon  as  shown  in  the  sketch;  electrical  contact  being  made  by  a 
terminal  plate,  Z,,  which  may  be  water-cooled.  Above  the  charge  of 
anthracite,  #,  is  placed  a  cover,  K,  of  some  good  heat-insulating 
material,  which  should  also  be  a  very  poor  conductor  of  electricity. 

A  view  of  this  furnace  in  operation  is  shown  in  Fig.  H7,2  which 
shows  clearly  the  arrangement  for  air-cooling  the  hearth,  the  massive 
end-walls,  and  the  electrical  connections  for  supplying  a  current  of 

*E.  G.  Acheson,  U.  S.  Patent  645,285,  March' 13, 1900.  Producing  graphite 
by  heating  anthracite,  etc.,  Electrochemical  Industry,  vol.  iv,  p.  42. 

Manufacturing  graphite,  British  patent  2,116,  of  1901,  by  O.  Imray,  of  the 
International  Acheson  Graphite  Company,  Electrochemist  and  Metallurgist, 
vol.  i,  p.  131. 

Manufacturing  of  artificial  graphite  from  charcoal,  J.  Weckbecker,  illustrated 
account,  Electro-chemical  Industry,  vol.  ii,  p.  244. 

Process  of  Making  Graphite,  E.  G.  Acheson,  U.  S.  patent,  711,031,  Electro- 
chemical Industry,  vol.  i,  p.  130. 

2  Reproduced  from  "Acheson  Graphite,"  a  catalogue  of  the  International 
Acheson  Graphite  Company. 


GRAPHITE  AND  CARBIDES 


285 


Ul 


UJ 


U4' 


286 


THE  ELECTRIC  FURNACE 


perhaps  15,000  or  20,000  amperes  to  the  furnace.  The  two  sets  of 
"  bus-bars,"  supplying  the  electric  current  to  a  number  of  furnaces, 
run  close  together  to  reduce  the  inductance  of  the  circuit,  and  the 
connection  to  one  electrode  is  carried  through  a  trench  under  the 
furnace. 

In  an  account  of  this  furnace  written  in  1902  by  Prof.  J.  W. 
Richards,1  it  is  said  to  be  30  ft.  long  and  formed  of  a  trough  2  ft. 
square,  lined  on  bottom  and  sides  with  blocks  of  compact  carborundum 
6  in.  thick.  Such  a  furnace  held  a  charge  of  about  6  tons  of  anthra- 
cite coal,  ground  to  the  size  of  rice,  and  this  was  graphitized  in  twenty 


FIG.  117. — Acheson  graphite  furnace. 


hours.  The  author  is  informed  by  the  Acheson  Graphite  Company 
that  no  refractory  lining  is  now  employed.  As  the  temperature  of 
formation  of  graphite  in  this  furnace  is  over  2,000°  C.,  it  must  be 
well  above  the  melting-point  of  ordinary  fire-clay  bricks,  and  a 
furnace  such  as  is  shown  in  Fig.  115  could  only  be  operated  if 
the  bottom  and  walls  were  composed  of  or  lined  with  some 
specially  refractory  material,  or  if  it  were  possible  to  leave  a  layer 
of  unconverted  anthracite  between  the  furnace  walls  and  the 
remainder  of  the  charge. 

1  The  Electrochemical  Industries  of  Niagara  Falls,  J.  W.  Richards,  Electro- 
chem.  Industry,  vol.  i,  p.  52. 


GRAPHITE  AND  CARBIDES  287 

By  increasing  the  cross- section  of  the  charge  there  would  be  less 
danger  of  the  walls  becoming  over-heated  and  the  graphitization 
of  the  central  portion  of  the  anthracite  might  still  be  effected  pro- 
vided the  electric  current  could  be  concentrated  on  this  portion 
instead  of  spreading  over  the  whole  of  the  cross-section.  At  the 
beginning  of  the  run  the  current  will  pass  almost  entirely  through 
the  core  of  carbon  rods,  and  when  this  conducting  core  is  augmented 
by  the  conversion  of  the  surrounding  anthracite  into  graphite,  its 
electrical  resistance  will  become  so  low  that  there  will  be  little 
tendency  for  the  current  to  pass  through  the  outer  portions  of  the 
charge.1 

The  electrode  furnace,2  Fig.  116,  resembles  the  graphite  furnace 
in  construction.  The  electrodes,  G,  or  other  articles  to  be  graphitized 
are  placed  in  piles  with  their  length  across  the  length  of  the  furnace, 
in  order  to  keep  the  electrical  resistance  as  high  as  possible.  They 
are  surrounded  by  broken  coke,  H,  which  has  a  moderately  high 
resistance,  so  that  most  of  the  heat  is  developed  in  the  parting  layers 
of  coke,  and  the  current  will  tend  to  pass  through  the  electrodes  in 
the  middle  of  the  furnace,  rather  than  through  the  outer  parts  of 
the  furnace  which  are  filled  with  the  broken  coke.  The  coke  will 
consequently  serve  as  a  jacket  to  retain  the  heat  and  prevent  the 
over-heating  of  the  walls  of  the  furnace.  The  dimensions  of  the  fur- 
nace depend  upon  the  size  of  the  electrodes,  which  can  be  as  much  as 
4  ft.  in  length.  A  cover  of  some  heat-insulating  substance  is  em- 
ployed, but  there  is  no  refractory  lining  of  carborundum,  etc.,  such 
as  was  described  in  the  patents  and  earlier  accounts. 

In  the  plant  of  the  International  Acheson  Graphite  Company 
at  Niagara  Falls  there  were  in  1906  six  furnaces  of  750  or  800  kw., 
and  ten  furnaces  of  1,600  kw.  The  total  power  employed  was  2,400 
kw.,  operating  one  furnace  of  each  size;  the  other  furnaces  being  in 
process  of  cooling,  emptying  or  recharging.  Each  furnace  takes 
about  half  a  day  to  heat,  and  four  or  five  days  to  cool.  The  total 

1  Compare  the  account  of  the  manufacture  of  soft  graphite,  where  an  outer 
portion  or  jacket  of  poorly  conducting  material  is  provided. 

2  Graphitizing  electrodes  and  other  carbonaceous  articles.    Patent  application, 
November  23,  1900,  with  illustration  of  electrode  furnace.     Electrochemist  and 
Metallurgist,  vol.  i,  1901,  p.  54. 

The  Ruthenburg  and  Acheson  Furnaces,  F.  A.  J.  FitzGerald,  Electrochemical 
Industry,  vol.  iii,  p.  417. 

E.  G.  Acheson,  U.  S.  patent  617,979,  January  17,  1899,  and  702,758;  June  17, 
1902,  Graphite  electrodes,  etc.  Mentions  current  and  size  of  furnace,  Electro- 
chemical Industry,  vol.  iv,  p.  42. 


288  THE  ELECTRIC  FURNACE 

output  of  the  plant  in  1906  being  nearly  6,000,000  Ib.  of  graphite. 
At  the  present  time  the  furnaces  take  4,000  E.H.P.  or  3,000  kw.  The 
electrical  current  is  supplied  at  a  voltage  of  2,200.  This  is  trans- 
formed to  a  current  at  200  volts,  and  the  voltage  is  further  lowered  by 
a  special  regulator  as  the  resistance  of  the  charge  decreases  during  the 
run,  so  that  all  the  available  power  can  be  applied  during  the  whole 
period.  J.  W.  Richards  stated  in  1902  that  for  the  electrode  fur- 
naces the  current  was  3,000  amperes  at  220  volts  to  begin  with,  and 
9,000  amperes  at  80  volts  at  the  end  of  the  operation.  The  current 
flowing  through  the  i,6oo-kw.  furnaces  must  be  about  twice  as  large 
as  this. 

The  conversion  of  amorphous  carbon  into  graphite  in  the  Acheson 
furnace  is  supposed  to  be  due  to  the  formation  and  subsequent 
decomposition  of  carbides  of  the  iron  and  other  metals  contained  in 
the  charge.  It  should  be  noted,  however,  that  the  amount  of  iron, 
aluminium,  calcium,  etc.,  contained  in  the  anthracite,  or  specially 
added  to  the  electrodes,  is  not  nearly  enough  to  combine  with  all 
the  carbon  to  form  carbides,  as  the  ash  or  impurity  in  anthracite 
varies  from  about  5  per  cent,  to  about  15  per  cent.,  and  the  amount 
of  iron-ore,  etc.,  which  is  added  to  the  electrode  material  in  order  to 
enable  the  manufactured  electrode  to  be  graphitized  is  only  2  per  cent, 
or  3  per  cent.  These  metals  may  be  expected  to  serve  for  more  than 
one  equivalent  of  carbon,  because  when  the  central  part  of  the 
furnace  has  become  hot  enough  to  dissociate  the  carbides  that  had 
formed  there,  the  volatilized  metals  will  escape  to  cooler  zones,  and 
will  again  form  carbides  with  the  carbon  at  that  point;  but  this 
explanation  hardly  accounts  for  all  the  carbon  that  is  graphitized 
through  the  agency  of  a  very  small  amount  of  added  metal,  although 
it  undoubtedly  explains  in  part  the  great  effect  of  these  added  bodies. 
Another  factor  will  be  found  in  the  consideration  that  dissociation 
and  reformation  of  the  carbides  is  always  taking  place  at  tempera- 
tures below  that  of  the  final  splitting  up  of  these  bodies,  and  that  in 
this  way  the  small  amount  of  metal  can  eventually  form  carbides 
with  all  the  carbon  in  the  furnace.  From  this  point  of  view,  time 
would  appear  to  be  an  essential  element  in  the  conversion  of 
carbon  into  graphite,  and  this  is  probably  the  case,  although  at 
the  high  temperature  of  the  graphite  furnace  these  molecular 
combinations  and  dissociations  will,  no  doubt,  take  place  with  great 
rapidity. 

The  character  and  uses  of  Acheson  graphite  are  fully  described 
in  a  series  of  pamphlets  issued  by  the  International  Acheson  Graphite 


GRAPHITE  AND  CARBIDES  289 

Company,1  and  in  other  published  papers,2  from  which  the  following 
points  may  be  summarized.  The  density  of  natural  graphite,  and 
of  the  Acheson  graphite  electrodes  is  2.2,  while  the  density  of  amor- 
phous carbon  electrodes  is  2.00.  The  graphite  electrodes  are  very 
pure,  containing  only  about  0.5  per  cent,  of  impurity.3  The  elec- 
trical conductivity  of  the  graphite  electrodes  is  about  four  times  that 
of  amorphous  carbon  electrodes,  and  the  cross-section  of  an  electrode 
can  be  proportionately  decreased  when  the  graphite  variety  is  em- 
ployed. Graphite  electrodes  are  found  to  have  a  far  longer  life  than 
those  of  amorphous  carbon  when  used  in  a  variety  of  electrolytic  pro- 
cesses; the  rate  of  corrosion  and  disintegration  being  very  much  less.4 
The  graphite  electrodes  have  another  valuable  property,  namely, 
that  of  easy  cutting  or  machining. 5  Electrodes  of  amorphous  carbon 
are  very  difficult  to  cut,  and  must  usually  be  molded  into  any  re- 
quired shape,  while  the  Acheson  graphite  electrodes  can  be  cut  with 
a  saw,  drilled,  threaded,  etc.,  so  that  all  kinds  of  shapes  can  readily  be 
prepared  from  rods  or  slabs  of  the  graphite.  In  particular,  it  is  possi- 
ble to  avoid  wasting  the  ends  of  electrodes,  by  the  use  of  threaded 
connections,  as  in  Fig.  48,  so  that  when  an  electrode  becomes  too  short 
for  further  use,  another  is  attached  to  its  outer  end,  and  the  lengthened 
electrode  can  be  fed  steadily  forward  into  the  furnace  or  electrolytic 
tank  without  interruption  of  the  process  or  waste  of  electrode.  In 
electrolytic  work,  composite  electrodes  are  often  used;  a  slab  of 
graphite  serves  as  the  working  electrode,  being  immersed  in  the 

1  Acheson   Graphite   Electrodes.     Pamphlet  by  The   International   Acheson 
Graphite  Company. 

2  Graphite  Electrodes  in  Electrometallurgical  Processes.     C.  L.  Collins,  Amer. 
Electrochem.  Soc.,  vol.  i,  p.  53. 

Uses  of  Acheson  Graphite  in  Metallurgical  Research,  W.  McA.  Johnson, 
Electrochemical  Industry,  vol.  ii,  p.  345. 

Graphite  electrodes  for  electric  furnace  work,  Electrochemical  Industry,  vol. 
iv,  p.  513.  The  Acheson  Graphite  Company  supplied  2,404,171  Ib.  of  graphitized 
electrodes  during  the  year  ending  July  i,  1906. 

3  The  pamphlet  on  Acheson  Graphite  Electrodes,  dated  1902,  states  that  the 
percentage  of  impurities  averages  about  i  part  in  1,000,  and  quotes  an  experiment 
in  which  their  electrodes  yielded  0.8  per  cent,  of  ash.     A  later  leaflet  on  Acheson 
Graphite  states  that  the  electrodes  contain  99.5  per  cent,  of  pure  graphitic 
carbon. 

4  Graphite  Electrodes  in  Electrolytic  Work,  by  C.  L.  Collins,  Electrochemical 
Industry,  vol.  i,  p.  26. 

5  Adaptability  of  Acheson  Graphite  Articles,  or  Ease  of  Machining.     Acheson 
Graphite  Company,  1904,  and  paper  by  C.  L.  Collins,  Electrochemical  Industry, 
vol.  ii,  p.  277. 

19 


290  THE  ELECTRIC  FURNACE 

electrolyte,  and  one  or  more  graphite  rods  which  are  threaded  into  the 
slab,  serve  to  support  it  and  lead  in  the  electric  current. 

Dr.  Acheson  succeeded,  in  the  year  1906,  in  producing  a  specially 
soft  variety  of  graphite  which  is  found  to  be  a  very  efficient  lubricant.1 

It  is  made  in  the  electric  furnace  from  anthracite  or  other  form  of 
amorphous  carbon  to  which  a  larger  amount  than  usual  of  carbide- 
forming  material  has  been  added.  Silica  is  preferred  for  this  purpose 
because  it  does  not  form  a  fusible  carbide.  The  amount  added  is  far 
greater  than  in  the  manufacture  of  graphite  electrodes,  or  in  the  con- 
version of  carbon  into  bulk  graphite,  but  it  is  less  than  would  be  re- 
quired for  making  a  carbide  with  the  whole  of  the  carbon.  The 
following  specific  case  is  contained  in  the  patent  application  :2 

"An  electric  furnace,  having  a  length  of  18  ft.  between  terminal  elec- 
trodes, was  provided  with  a  starting  core  consisting  of  a  graphite  rod  7/8 
in.  in  diameter.  The  active  zone,  18  in.  in  diameter,  surrounding  this 
core,  was  filled  with  a  mixture  of  carbonaceous  material  and  carbide- 
forming  oxide.  The  materials  used  in  this  specific  instance  were  anthra- 
cite coal,  ground  to  pass  through  a  i/2-in.  mesh,  mixed  with  sand,  in  the 
proportion  of  65  per  cent,  coal  and  35  per  cent,  sand,  the  ash  contained 
in  the  coal  being  calculated  as  a  part  of  the  sand-content  of  the  mixture. 
Completely  surrounding  the  active  zone  above  referred  to  was  disposed 
a  mixture  of  anthracite  coal  and  sand,  in  the  proportion  of  i  part  coal  to 
2  parts  of  sand,  this  mixture  having  a  much  higher  resistance  than  that 
in  the  active  zone,  and  serving  as  an  effective  heat-retainer.  The  furnace 
being  charged  in  this  manner  the  electric  current  was  turned  on,  and  at 
the  beginning  registered  79  volts  and  75  kw.  After  2  hours,  the  register 
showed  203  volts  and  200  kw.,  and  after  9  1/2  hours  showed  135  volts 
and  800  kw.  The  register  at  the  end  of  15  hours  still  showed  800  kw., 
while  the  volts  had  dropped  to  70,  as  the. result  of  decreased  internal 
resistance,  due  to  the  formation  of  graphite.  When  cold  the  furnace 
was  opened  and  962  Ib.  of  soft,  unctuous  and  non-coalescing  graphite  were 
removed  from  the  active  zone." 

Soft  or  unctuous  graphite  is  supplied  in  admixture  with  grease 
under  the  name  of ' '  Gredag. "  "  D eflocculated  graphite " 3  is  a  variety 
of  graphite  that  has  been  so  finely  divided  by  a  special  process  that 
it  will  remain  permanently  suspended  in  oil  or  in  water.  It  is  sup- 
plied mixed  with  oil  under  the  name  "oildag,"  and  mixed  with  water 

1  Soft  Graphite,  Electrochemical  Industry,  vol.  iv,  pp.  343  and  502. 

2  Soft    Graphite.     E.    G.  Acheson,  U.    S.    patent  836,355,  Electrochemical 
Industry,  vol.  iv,  p.  502. 

3  E.  G.  Acheson,  "  Deflocculated  Graphite."  Trans.  Am.  Electrochem.  Soc., 
xii,  1907,  p.  29. 


GRAPHITE  AND  CARBIDES  291 

under  the  name  "aquadag."     These  mixtures,  after  dilution  with  oil 
and  water  respectively  form  very  effective  lubricants. 

Kryptol,1  a  material  specially  prepared  for  use  as  a  resistor  in  elec- 
tric heaters  and  furnaces,  appears  to  consist  of  a  mixture  of  graphite 
and  amorphous  carbon,  in  grains  of  nearly  uniform  size.  This  mate- 
rial can  be  given  a  higher  or  lower  electrical  conductivity  by  varying 
the  proportion  of  graphite  and  amorphous  carbon,  and  by  changing 
the  size  of  the  grains  of  which  it  is  formed. 

CARBORUNDUM 

Carbide  of  silicon  SiC  was  discovered  independently  by  three  in- 
vestigators, E.  G.  Acheson,  A.  and  H.  Cowles  and  H.  Moissan.  The 
brothers,  E.  H.  and  A.  C.  Cowles,  appear  to  have  discovered  the  com- 
pound in  an  electric  furnace  in  1885,  but  they  did  not  know  its  com- 
position and  their  work  was  interrupted.  Moissan  produced  this 
carbide  in  1891  but  did  not  publish  his  work  until  later.  E..G. 
Acheson,  only  shortly  after  Moissan's  discovery,  made  the  experiment 
of  heating  carbon  and  clay  in  an  electric  furnace,  and  obtained  a  mass 
of  bright  blue  crystals,  which  he  at  first  supposed  to  consist  of  carbon 
and  alumina,  hence  the  name  of  carborundum.  Further  experiments 
showed  that  these  crystals  of  carborundum  could  be  obtained  by 
heating  sand  and  carbon  and  that  they  were  the  carbide  of  silicon  SiC. 
The  discovery  of  this  carbide  by  E.  G.  Acheson  in  1891  2  was  the  first 
step  leading  to  the  considerable  industries  at  Niagara  Falls,  with  which 
he  is  now  associated. 

The  formation  of  silicon  carbide  may  be  represented  by  the  follow- 
ing equation: 


60        36    40        56 

This  carbide  is  amorphous  when  it  is  first  formed,  but  on  being 
more  strongly  heated  it  crystallizes  and  is  then  known  as 
carborundum. 

Carborundum  is  produced  by  heating  a  mixture  of  coke,  silicious 
sand,  sawdust  and  salt  in  an  electric  furnace  such  as  is  shown  in  opera- 
tion in  Fig.  1  1  8,  and  diagrammatically  in  Fig.  8.  The  charge  is  made 

1  Kryptol,  Electrochemical  Industry,  vol.  ii,   pp.  333  and  463,  vol.  iii,  pp.  5, 
127,  and  157,  and  vol.  iv,  pp.  148,  210,  250,  2q6,  and  344. 

2  E.  G.  Acheson,  "Discovery  and  Invention."     The  Electric  Journal,  Pitts- 
burgh, 1906. 


292 


THE  ELECTRIC  FURNACE 


approximately  in  the  following  proportions: — Coke  34.2  per  cent., 
sand  54.2  per  cent.,  sawdust  9.9  per  cent.,  and  salt  1.7  per  cent.;  the 
sand  and  the  carbon  in  the  charge  being  nearly  in  the  proportion  in- 
dicated by  the  equation.  The  sawdust  contained  in  the  charge  has 
the  effect  of  rendering  it  more  porous  and  of  allowing  the  gases  to 
escape  more  freely,  while  the  salt  is  found  to  facilitate  the  running  of 
the  furnace. 

Later  accounts  show  a  larger  proportion  of  salt  and  a  smaller 
amount  of  sawdust  in  the  charge.  J.  W.  Richards,1  writing  in  1902, 
and  describing  a  furnace  of  750  kw.,  says,  "The  total  contents  of  the 
furnace  are  about  1,000  Ib.  of  carbon  core,  and  the  mixture  reduced 


FIG.  118. — Carborundum  furnace. 

represents  3.5  tons  of  carbon  mixed  with  6  tons  of  silica  sand  and  1.5 
tons  of  salt,  producing  in  a  36-hour  run  between  3  and  4  tons  of  com- 
mercial carborundum,  in  crystals,  outside  of  which  is  a  quantity  of 
light  green  amorphous  carborundum,  fully  reduced,  but  uncrystal- 
lized,  and  outside  of  this  the  unchanged  mixture."  E.  G.  Acheson's 
United  States  patent  No.  560,291,  May  19,  1896,2  for  a  carborundum 
furnace  of  1,000  kw.  specifies  a  mixture  of  "20  parts  (by  weight)  of 

1  The  Electrochemical  Industries  of  Niagara  Falls,  J.  W.  Richards,  Electro- 
chemical Industry,  vol.  i,  p.  50. 

2E.   G.  Acheson,  U.   S.  patent  560,291,  Electrochemical  Industry,  vol.  v, 
p.  70. 

An  earlier  patent,  492,767,  February  28,  1893,  is  abstracted  in  vol.  v,  p.  36. 


GRAPHITE  AND  CARBIDES  293 

finely  divided  coal  or  coke,  29  parts  of  sand,  5  parts  of  common  salt, 
and  2  parts  of  sawdust." 

The  mixture  of  coke,  sand,  etc.,  is  not  a  good  conductor  of  elec- 
tricity, and  the  heating  must  therefore  be  effected  by  means  of  a 
core  of  broken  coke,  marked  E  in  Fig.  8,  which  extends  between 
the  two  electrodes,  C  and  D,  and  serves  as  a  resistor,  carrying  the 
electric  current  and  heating  the  surrounding  charge.  The  coke 
for  making  up  the  charge  is  ground  to  a  powder,  but  the  coke  for 
the  resisting  core  is  in  small  pieces  about  1/4  or  3/8  in.  in  size, 
from  which  the  dust  has  been  removed.  The  core  is  circular  in 
section  and  is  built  up  by  hand  after  the  furnace  has  been  half  filled, 
a  packing  of  fine  carbon  powder  serves  to  make  good  electrical  con- 
tact between  the  core  and  the  carbon  electrodes.  The  electrodes 
consist  of  a  number  of  square  rods  of  carbon  or  graphite,  which  are 
held  by  heavy  bronze  holders  F  and  G.  Electrical  contact  with 
the  carbon  rods  being  rendered  more  perfect  by  a  series  of  copper 
strips,  indicated  by  white  lines  in  Fig.  8,  which  are  laid  between  the 
rows  of  carbon  rods  and  are  connected  to  the  bronze  holders  or 
directly  to  the  cables  from  the  bus-bars.  The  general  construction 
of  the  furnace  is  similar  to  that  of  the  graphite  furnaces,  the  end 
walls  and  bottom  of  the  furnace  being  permanent,  while  the  side 
walls  are  loosely  built,  allowing  the  carbonaceous  gases  to  escape 
and  burn  as  shown  in  Fig.  1 18,  and  are  taken  down  between  the  opera- 
tions to  allow  of  emptying  and  refilling  the  furnace.  The  electrical 
equipment  is  similar  to  that  of  the  graphite  furnaces,  but  there  are 
more  electrical  units  provided,  as  three  75o-kw.  furnaces  and  one 
i.ooo-kw.  furnace  can  be  operated  at  once. 

The  carborundum  furnace  has  been  described  by  F.  A.  J.  Fitz- 
Gerald1  who  gives  a  scale  drawing  of  a  75o-kw.  furnace.  .The  furnace 
was  1 6.£  ft.  long,  6  ft.  wide,  and  5.5  ft.  high  inside,  and  had  a 
core  of  coke,  16  ft.  long  and  20  in.  in  diameter.  In  the  patent 
just  referred  to  a  furnace  of  1,000  kw.  was  stated  to  have  a  core 
8  ft.  long  and  10  in.  in  diameter,  composed  of  grains  3/16  in.  in  diam- 
eter, of  coked  bituminous  coal. 

At  the  end  of  the  operation  the  carborundum  is  found  in  a  cylin- 
drical crystallized  mass  surrounding  the  core,  and  around  the  car- 
borundum is  a  layer  of  uncrystallized  carbide  which  has  been  called 
carborundum  fire-sand.  In  the  furnace  figured  by  FitzGerald 
the  carborundum  cylinder  is  50  in.  in  diameter.  The  layer  of  fire- 

1  The  Carborundum  Furnace,  F.  A.  J.  FitzGerald,  Electrochemical  Industry, 
vol.  iv,  1906,  p.  53. 


294 


THE  ELECTRIC  FURNACE 


sand  is  often  i  in.  or  1.5  in.  in  thickness.  The  grains  of  coke 
composing  the  core  have  become  partly  graphitized.  They  can  be 
used  again,  and  their  use  will  be  better  in  general  than  that  of  fresh 
coke  as  the  resistance  of  the  core  will  be  less  variable.  The  tem- 
perature of  the  furnace  is  highest  in  the  middle,  that  is  just  around 
the  core,  and  the  inner  part  of  the  carborundum  may  be  heated  above 
its  dissociation  temperature  and  be  converted  into  graphite,  the 
silicon  being  volatilized  and  driven  into  the  cooler  parts  of  the 
furnace.  The  accidental  formation  of  graphite  in  this  way  led  to 
its  regular  manufacture  in  the  Acheson  furnace.  The  temperature 
of  the  carborundum  furnace  has  been  measured  by  Messrs.  Tucker 
and  Lampen1  who  find  the  dissociation  temperature  of  carborundum, 
and  therefore  the  hottest  part  of  the  furnace  to  be  2,220°  C.  Mr. 
Saunders2  has  recently  repeated  these  tests  and  practically  confirms 
the  results  obtained  by  Tucker  and  Lampen,  placing  the  formation 


FIG.  119. — Improved  carborundum  furnace. 

of  amorphous  silicon  carbide  at  1,600°  C.,  that  of  carborundum  at 
1,840°  C.  and  its  decomposition  into  carbon  and  silicon  at  2,240°  C. 
The  carborundum  furnace  shown  in  Figs.  8  and  118  has  been 
improved  recently  by  Mr.  Tone,  in  order  to  facilitate  its  operation. 
The  temporary  side-walls  of  the  furnace,  which  had  to  be  built  up 
fresh  for  every  charge,  have  been  replaced  by  a  series  of  iron  frames 
lined  with  bricks  as  shown  in  Fig.  up.3  These  frames  are  portable 
and  can  be  removed  by  an  overhead  crane  at  the  end  of  one  operation 
and  reassembled  on  an  empty  furnace  in  a  very  short  time.  The 

1  S.  A.  Tucker  and  A.  Lampen,  "The  Measurement  of  Temperature  in  the 
formation  of  Carborundum,"  Jour.  Am.  Chem.  Soc.,  xxviii,  1906,  p.  853. 

2L.  E.  Saunders,  "Temperature  Measurements  on  the  Silicon  Carbide  Fur- 
nace," Trans.  Am.  Electrochem.  Soc.,  xxi,  1912,  p.  438. 

8  F.  J.  Tone,  U.  S.  patent  800,515,  1905. 


GRAPHITE  AND  CARBIDES  295 

furnace-hearth  is  also  provided  with  tubes  for  water-cooling  (not 
shown  in  the  figure),  as  otherwise  the  brick- work  was  found  to  become 
so  hot  as  to  become  an  electric  conductor  and  carry  part  of  the  cur- 
rent which  should  have  been  confined  to  the  materials  of  the  charge. 

The  electrical  equipment  of  the  Carborundum  Company  amounts 
to  7,000  h.p.  The  furnaces  are  of  1,000  to  2,000  h.p.  each,  and  the 
electrical  supply  is  arranged  in  a  series  of  units  of  1,000  or  2,000 
h.p.  Each  electrical  unit  can  be  connected  to  any  one  of  a  series 
of  five  furnaces,  one  of  which  is  in  operation  while  the  others  are 
being  cooled,  discharged  or  loaded.  The  electrical  power  is  received 
at  a  voltage  of  2,200  with  a  frequency  of  25  cycles,  and  supplies 
transformers  of  1,600  kw.  The  secondary  current  from  these 
transformers  has  a  voltage  of  about  180  and  this  can  be  regulated 
within  considerable  limits,  by  cutting  out  some  of  the  primary 
windings.  This  regulation  (see  Fig.  49)  is  effected  by  a  system  of 
oil  switches  operated  electrically  from  a  controller.  The  trans- 
formers are  placed  close  to  the  furnaces  they  are  to  supply,  but  the 
voltage  is  controlled,  by  one  operator,  from  a  central  room  where 
the  electric  meters  are  placed. 

The  consumption  of  energy  per  pound  of  carborundum  was 
given  by  J.  W.  Richards  as  3.8  kw.  hours.  The  output  in  1905 
was  5,596,000  Ib.  Its  main  use  is  as  an  abrasive,  being  used  instead 
of  emery;  it  is  made  into  wheels,  sticks,  hones,  etc.,  the  masses  of 
carborundum  being  crushed,  washed  and  graded  into  powders  of 
varying  degrees  of  fineness.  These  powders  are  usually  cemented 
together  to  form  the  carborundum  articles  by  molding  with  kaolin 
and  feldspar  and  firing  in  a  kiln,  but  a  method  has  been  devised1 
for  making  solid  blocks  of  carborundum  by  cementing  the^  grains 
together  with  thin  glue,  and  then  heating  in  the  electric  furnace  to 
the  temperature  of  the  formation  of  carborundum.  The  grains 
become  firmly  cemented  together.  The  uses  of  carborundum  and 
carborundum  fire-sand  as  refractory  materials  have  been  referred 
to  in  Chapter  IV.  The  Carborundum  Company  have  a  plant  at 
Duesseldorf  in  Germany  for  the  manufacture  of  carborundum  for 
the  European  market.2 

Moissanite. — This  is  a  natural  form  of  carborundum  which  was 
discovered  by  G.  F.  Kunz,3  in  the  Canon  Diablo  Meteorite. 

1  F.  A.  J.  FitzGerald,  U.  S.  patent  650,234,  Electrochemical  Industry,  vol.  v, 
p.  70. 

2  Electrochemical  Industry,  vol.  iv,  p.  348. 

3  G.  F.  Kunz,  Trans.  Am.  Electrochem.  Soc.,  xii,  1907,  p.  45. 


296  THE  ELECTRIC  FURNACE 

Silundum1  is  a  compact  form  of  carborundum  formed  by  the 
conversion  of  carbon  into  silicon  carbide  by  impregnation  with 
silicon  vapor.  This  is  accomplished  in  a  furnace  like  that  used  for 
the  production  of  carborundum;  the  carbon  articles  to  be  converted 
into  silundum  being  packed  in  a  suitable  charge.  The  charge  may 
be  composed  of  a  mixture  of  silica  and  carborundum,  according  to 
the  equation: 


or  more  usually  of  a  mixture  of  sand  and  coke  according  to  the 
equation: 


The  furnace  is  provided  with  the  usual  resisting  core  of  broken 
carbon,  and  the  articles  to  be  converted  are  separate  from  the  core. 
The  furnace  is  run  in  the  usual  manner  to  form  silicon.  This  is  in 
the  form  of  vapor  and  penetrates  the  carbon  articles,  forming  a  com- 
pact semi-metallic  product  known  as  silundum.  The  carbon  is 
converted  into  silundum  beginning  at  the  outside  and  continuing 
to  a  depth  which  depends  on  the  length  of  the  operation;  so  that 
there  will  frequently  be  a  core  of  unchanged  carbon  within  the  sheath 
of  silundum. 

Silundum  will  probably  be  of  value  for  electrical  resistors,  on 
account  of  its  ability  to  withstand  oxidation,  when  heated  in  air  to 
temperatures  as  high  as  1,200°  C.  Resisting  grids  have  been  made 
of  rods  of  silundum  fused  together  by  means  of  silicon. 

Siloxicon.  —  This  term  covers  a  series  of  compounds  of  silicon, 
carbon  and  oxygen,  having  the  general  formula  Si^C^O,  which 
are  produced  in  the  electric  furnace  by  heating  a  mixture  of  carbon 
and  silica,  in  which  there  is  not  enough  carbon  to  form  carbide  of 
silicon  with  the  whole  of  the  silica.  This  material  was  patented  by 
Dr.  Acheson  in  1902.  2  The  proportions  indicated  in  the  patent 
are  one  part  of  powdered  carbon  to  two  parts  of  powdered  silica. 
The  mixture  must  not  be  heated  to  the  temperature  of  formation  of 
carborundum,  because  at  that  temperature  siloxicon  dissociates  into 
carborundum,  silicon  and  carbon  monoxide.  On  account  of  the 
danger  of  over-heating,  it  is  desirable  to  employ  a  furnace  with  a 
number  of  cores,  moderately  heated;  this  arrangement  enabling  a 

1  S.  A.  Tucker,  Trans.  Am.  Electrochem.  Soc.,  vol.  xvi,  1909,  p.  207. 

2  Refractory  Material,  E.  G.  Acheson,  U.  S.  patent  722,793.     Electrochemical 
Industry,  vol.  i,  p.  287. 


GRAPHITE  AND  CARBIDES 


297 


large  volume  of  the  charge  to  be  heated  to  a  nearly  uniform  tem- 
perature, and  thus  giving  a  better  yield  of  the  siloxicon.  Such  a 
furnace  is  shown  diagrammatically  in  Fig.  120,  the  core  consisting 
of  a  large  number  of  rods  of  graphitized  carbon  connected  together 
by  being  fitted  into  blocks  of  graphite.  In  the  furnace  shown  the 
rods  are  arranged  partly  in  series  and  partly  in  parallel,  one-third  of 
the  entire  current  passing  through  each  rod.  The  graphite  blocks 
are  shown  supported  on  a  layer  of  some  refractory  material  on  the 


FIG.  1 20. — Multiple  core  furnace. 

bottom  of  the  furnace.  Siloxicon,  the  most  usual  formula  for  which 
is  Si2C2O,  is  a  loose  powdery  gray-green  amorphous  material,  and 
can  readily  be  removed  from  the  furnace  after  taking  down  the  side 
walls,  by  raking  it  out  from  between  the  cores.  Such  a  furnace 
might  be  employed  for  the  manufacture  of  amorphous  silicon 
carbide,  but  could  not  be  used  for  the  production  of  the  crystallized 
carborundum,  as  the  system  of  cores  would  be  destroyed  after  each 
operation.  Siloxicon  forms  a  valuable  refractory  material,  and  its 
properties  and  uses  have  been  described  in  Chapter  IV. 


298  THE  ELECTRIC  FURNACE 

CALCIUM  CARBIDE 

When  lime  is  heated  with  carbon  in  the  electric  furnace  to  a  high 
temperature  (about  2,000°  C.)  the  li«ie  is  reduced  by  the  carbon  to 
the  metal  calcium.  This  combines  with  additional  carbon  to  form 
calcium  carbide,  which  is  fluid  at  the  temperature  of  the  furnace. 
The  reaction  may  be  expressed  generally  by  the  equation: 


56        36      64        28 

The  production  of  calcium  carbide,  in  this  way,  was  discovered  in  the 
year  1892  by  T.  L,  Willson,  whose  name  is  associated  with  the  de- 
velopment of  the  calcium-  carbide  industry,  and  independently  by 
H.  Moissan.1 

The  reaction  between  lime  and  carbon,  forming  calcium  carbide, 
begins  at  temperatures  as  low  as  1^500°  C.,2  but  a  temperature  of 
about  i,  800°  C.,3  is  needed  to  melt  the  carbide,  and  a  temperature  at 
least  as  high  as  this  would  have  to  be  maintained  for  the  proper 
operation  of  the  furnace.  Moissan  gives  3,000°  C.  as  the  temperature 
of  formation  of  calcium  carbide,  but  although  the  temperature  of  the 
arc  is,  no  doubt,  as  high  as  this,  the  resulting  carbide  is  probably  not 
heated  much  above  2,000°  C.  When  calcium  carbide  is  heated  con- 
siderably above  its  melting-point,  it  dissociates  with  the  formation  of 
calcium  vapor  and  graphite.  The  calcium  escapes  and  is  oxidized 
outside  the  furnace,  and  the  carbide  remaining  in  the  furnace  is 
consequently  of  a  lower  quality  on  account  of  the  admixture  of 
graphite.  Carbide  which  has  been  over-heated  in  this  way,  is  said  to 
be  "burnt."  Carbide  furnaces  are  in  general  operated  by  the  elec- 
tric arc,  and  are  of  two  kinds  —  "Ingot"  furnaces  and  "Tapping" 
furnaces. 

INGOT  FURNACES 

The  Willson  furnace,  shown  in  Fig.  7,  was  an  early  form  of  ingot 
furnace.  An  iron  box,  A,  mounted  on  -a  car,  had  a  rammed  carbon 

1  V.  B.  Lewes  gives  an  account  of  the  history  of  this  discovery  in  his  book  on 
"Acetylene."     T.  L.  Willson  appears  to  be  the  original  discoverer. 

2  M.  de  K.  Thompson,  "Preparation  and  Properties  of  Calcium  Carbide," 
Trans.  Am.  Electrochem.  Soc.,  xvi,  p.  197. 

C.  A.  Hansen,  "Calcium  Carbide,"  Electrochem.  and  Met.  Ind.,  vol.  vii, 
1909,  p.  427. 

3  A.  Lampen,  Jour.  Am.  Chem.  Soc.,  vol.  xxviii,  1906,  p.  846. 


GRAPHITE  AND  CARBIDES  299 

bottom,  D,  which  formed  one  electrode.  The  other  electrode,  B  C, 
was  movable,  and  was  gradually  raised  as  the  mass  of  carbide  accumu- 
lated in  the  furnace.  The  charge  of  coke  or  anthracite  and  lime  was 
fed  in  so  as  to  cover  the  arc,  and  the  resulting  carbon  monoxide  burned 
above  the  charge.  A  furnace  of  this  type  would  take  about  2,000 
amperes  at  75  volts;  the  position  of  the  electrode  being  controlled 
automatically,  so  as  to  maintain  a  constant  current.  When  a  suf- 
ficient amount  of  carbide  had  been  formed,  the  box  was  removed  and 
allowed  to  cool.  The  contents  were  then  dumped  out  and  the  loose 
charge  and  the  half-formed  carbide  were  separated  from  the  compact 
and  well-melted  carbide.  In  such  a  furnace  the  loose  charge  forms 
the  working  lining,  and  in  order  to  protect  the  iron  box  from  the  heat 
of  the  molten  carbide  a  considerable  amount  of  charge  is  utilized  in 
this  way,  and  is  not  converted  into  carbide,  but  can  be  used  over 
again.  In  some  cases,  as  much  as  4  tons  of  material  were  needed  for 
i  ton  of  carbide.  In  this  furnace  the  current  has  to  pass  through 
a  mass  of  molten  carbide  (to  reach  the  bottom  electrode),  and  as  this 
accumulates  and  solidifies,  electrical  energy  is  wasted  in  causing  the 
current  to  pass  through  the  partly  solidified  mass. 

Improved  Willson  Furnace. — A  more  recent  form  is  shown  in  Fig. 
121.  In  this  furnace  two  or  more  movable  electrodes  are  used,  the 
current  entering  through  one  and  leaving  through  the  other,  and 
forming  an  arc  between  each  electrode  and  the  molten  carbide.  In 
this  way  the  current  has  not  to  pass  through  any  considerable  accumu- 
lation of  carbide.  The  furnace  consists  of  a  sheet-iron  box,  about 
6  ft.  high,  stiffened  with  tee-iron  and  standing  on  a  small  iron  truck. 
Two  electrodes  are  used,  each  consisting  of  a  number  of  round  graphite 
rods  threaded  and  screwed  into  a  water-cooled  iron  holder,  which  is 
shown  in  detail  in  Fig.  39.  The  cables  that  bring  the  current  are  con- 
nected to  the  upper  end  of  this  holder,  so  as  to  be  removed  from  the 
heat  of  the  furnace.  The  rods  connecting  the  upper  and  lower  ends 
of  the  holder  are  iron  pipes  which  carry  the  cooling  water.  The  two 
holders  are  connected  together  mechanically  at  their  upper  ends, 
but  are  insulated  from  each  other  by  pieces  of  vulcanized  fiber,  and 
the  whole  is  lifted  by  a  chain  connected  to  an  electric  motor  which  is 
operated  automatically  so  as  to  maintain  a  constant  electric  current 
through  the  furnace. 

The  charge  consists  of  burnt  lime,  and  coke  or  anthracite.  These 
are  crushed  to  1/8  or  3/16  in.  and  are  then  mixed  and  supplied  to  the 
furnace  from  two  bins.  The  charge  passes  down  inclined  shoots 
which  are  provided  with  gates  to  regulate  the  supply  to  the  furnace. 


300 


THE  ELECTRIC  FURNACE 


The  charge  in  the  furnace  is  kept  about  two  feet  above  the  level 
of  the  arc,  and  occasionally  requires  to  be  poked  to  cause  it  to 
settle;  holes  being  left  in  the  front  of  the  iron  box  through  which 
the  charge  can  be  poked  when  necessary  to  break  up  any  scaffolds 
or  obstructions.  The  gases  are  led  away  through  a  flue  from  the 
upper  part  of  the  furnace. 


FIG.  121. — Willson  carbide  furnace. 


The  power  used  for  one  furnace  is  about  3,500  amperes  at  75  volts, 
or  300  h.p.  A  charge  of  lime  and  coke  weighing  1,400  Ib.  is  smelted 
in  about  13  hours,  yielding  800  Ib.  of  marketable  carbide.  The  box 
is  then  taken  out  of  its  place,  and  allowed  to  cool  before  dumping  the 
contents:  a  fresh  box  being  substituted.  The  electrodes,  when  made 
of  graphite,  last  130  hours;  amorphous  electrodes  last  100  hours  and 
cost  less,  but  are  more  difficult  to  thread. 


GRAPHITE  AND  CARBIDES 


301 


The  Bullier  ingot  furnace,  Fig.  122,  is  built  of  brickwork,  with 
a  drop-bottom  which  opens  to  discharge  the  ingot  of  carbide.  In 
the  furnace  shown  in  the  figure,  there  is  only  one  movable  electrode, 
as  in  the  early  Willson  furnace;  the  drop-bottom  forming  the  other 
electrode.  The  furnace  could,  however,  be  operated  with  two 
electrodes  as  in  Fig.  121;  no  connection  being  made  to  the  bottom. 
After  an  ingot  has  been  formed  in  this  furnace,  it  must  be  left  for 


FIG.  122. — Bullier  carbide  furnace. 

some  time  to  cool  before  it  can  be  dumped;  but  on  the  other  hand, 
the  heat  of  the  molten  ingot  serves  to  convert  some  of  the  surround- 
ing charge  into  carbide,  and  the  heat  that  passes  into  the  brickwork 
is  not  all  lost,  but  is  restored,  in  part,  to  the  charge  in  the  next 
operation. 

In  the  Willson  and  Bullier  furnaces  the  electrodes  are  gradually 
raised  as  the  ingot  of  carbide  is  built  up;  but  in  some  other  furnaces 


302 


THE  ELECTRIC  FURNACE 


the  electrodes  are  stationary,  and  the  furnace  is  lowered  as  the 
ingot  grows.  The  most  satisfactory  appliances  working  on  this 
principle  are  the  rotary  furnaces,  in  which  the  smelting  chamber 
forms  the  rim  of  a  large  vertical  wheel  or  spool.  The  smelting 
takes  place  at  one  part  of  the  rim,  and  the  block  of  carbide  is  carried 
away,  as  fast  as  it  is  formed,  by  the  slow  rotation  of  the  wheel. 
The  Horry  furnace,  Fig.  123,  is  of  this  kind.  It  consists  of  a 


Sectip.n  A»~ 
FIG.  123. — Horry  carbide  furnace. 

wheel  about  8  ft.  in  diameter  and  3  ft.  wide,  having  an  annular 
chamber  of  rectangular  cross-section  in  which  the  smelting  operation 
takes  place.  This  chamber  is  enclosed  permanently  on  three  sides 
by  the  hub,  and  sides  of  the  wheel,  but  the  fourth  side  is  closed  by 
movable  plates.  These  plates  are  taken  off  at  the  upper  part  of 
the  wheel  where  the  smelting  is  taking  place  and  where  the  solidified 
carbide  is  being  removed. 

The  electric  current  is  supplied  to  two  electrodes,  E  E,  which  are 


GRAPHITE  AND  CARBIDES  303 

located  in  a  hopper  into  which  the  charge  of  coke  and  lime  is  fed. 
An  arc  is  maintained  between  the  ends  of  these  electrodes,  and  the 
resulting  molten  carbide  forms  at  the  point  C,  and  is  carried  down- 
ward by  the  slow  movement  of  the  wheel;  ultimately  solidifying  and 
fomimg  the  ring  of  carbide  C  D.  The  cover  plates  are  attached  to 
the  wheel  at  P,  and  after  the  rotation  has  brought  them  to  Q  they 
are  removed  to  allow  the  solidified  carbide  to  be  broken  away.  A 
furnace  of  this  type  may  use  4,000  amperes  at  75-80  volts,  and  will 
make  a  complete  revolution  in  about  24  hours. 

The  Bradley  furnace  is  of  the  same  type,  but  has  only  one  carbon 
electrode;  the  arc  being  formed  between  this  and  the  molten  carbide. 
The  return  connection  for  the  electric  current  is  made  by  a  number 
of  copper  plugs,  in  the  inner  wall  of  the  rim,  which  make  electrical 
connection  with  the  block  of  carbide,  and  connect  it  to  a  commutator 
on  the  shaft  of  the  furnace.  Some  difficulty  was  found  with  this 
arrangement,  and  in  a  later  form  the  electrical  connection  was  made 
to  the  whole  of  the  smelting  chamber,  which  was  lined  with  graphite, 
and  in  this  way  the  current  was  led  to  the  carbide. 

TAPPING  FURNACES 

In  the  carbide  furnaces  already  described  the  product  is  allowed 
to  solidify  as  fast  as  it  is  formed  and  builds  up  an  ingot  which  must 
be  allowed  to  cool  and  then  removed  from  the  furnace.  It  is  also  nec- 
essary to  mechanically  remove  from  the  ingot  a  quantity  of  half- 
formed  carbide  before  it  can  be  broken  up  and  packed  for  market. 
These  difficulties  can  be  avoided  by  tapping  the  molten  carbide  from 
the  furnace,  and  many  furnaces,  which  are  known  as  "tapping"  or 
"running"  furnaces,  are  operated  in  this  manner. 

The  furnace  shown  in  Fig.  1 24  may  be  taken  as  a  typical  tapping 
furnace.  It  consists  of  a  circular  iron  case,  about  10  ft.  in  diameter, 
having  a  rammed  filling  of  carbon,  C,  about  2  ft.  thick,  which  is 
connected  with  one  pole  of  the  electrical  supply.  A  lining,  B,  about 
9  in.  thick,  is  built  of  fire-bricks  which  are  dipped  in  tar  before  being 
laid,  and  an  inner  lining,  L,  is  formed  automatically  of  the  loose 
charge  and  half-fused  carbide  produced  in  the  operation  of  the 
furnace. 

The  electrode,  E}  is  50  in.  in  diameter  and  8  ft.  long.  It  is  built 
up  of  a  number  of  round  electrodes  which  are  assembled  in  a  common 
holder  and  have  carbon  paste  rammed  around  and  between  them  to 
form  the  large  electrode.  A  sheet-iron  jacket  is  used  to  protect 


304 


THE  ELECTRIC  FURNACE 


the  upper  part  from  oxidation.  The  furnace  is  provided  with  a 
spout  and  tapping-hole  which  is  closed  during  the  operation  and 
opened  periodically  to  draw  off  the  molten  carbide.  The  heat  is 
produced  in  this,  as  in  most  other  carbide  furnaces,  by  an  arc  between 
the  electrode,  E,  and  the  molten  carbide. 

A  furnace  of  the  size  indicated  will  take  from  20,000  to  30,000 
amperes  at  about  50  volts  which  is  equivalent  to  about  1,200  kw. 


ft-  ".-*'-  ^^ 


FIG.  124. — Tapping  carbide  furnace. 

Large  tapping  furnaces  operating  with  three-phase  current  and 
having  three  electrodes  (see  Fig.  113)  will  use  as  much  as  2,000- 
3,000  h.p.1 

The  Helfenstein  furnace2  for  calcium  carbide  is  intended  for  three- 
phase  current.     It  consists  of  three  smelting  chambers,  built  close 

1  R.  A.  Witherspoon,  "Manufacture  of  Calcium  Carbide,"  Jour.  Soc.  Chem. 
Ind.,  xxxii,  1913,  p.  113.     The  author  is  indebted  to  Mr.  Witherspoon  for  par- 
ticulars of  the  tapping  furnace,  Fig.  124,  and  for  some  other  information  contained 
in  this  section. 

2  R.  Taussig,  Faraday  Soc.,  v,  1910,  p.  254;  Soc.  Chem.  Ind.,  xxix,  1910,  p. 
435;  Met.  and  Chem.  Eng.,  x,  1912,  p.  686. 


GRAPHITE  AND  CARBIDES  305 

together,  and  provided  with  a  rammed  carbon  electrode,  extending 
under  all  the  chambers.  Each  chamber  has  one  movable  electrode 
which  is  connected  to  one  lead  of  the  three-phase  supply;  the  common 
electrode,  which  forms  the  bottom  of  the  three  furnaces,  serving  as 
the  neutral  point  of  the  system.  Provision  is  made  for  sealing  the 
top  of  the  furnace  so  that  the  carbon  monoxide  is  obliged  to  escape 
through  flues  in  the  brickwork,  and  can  be  employed  for  burning 
limestone  or  preheating  the  charge. 

Comparing  tapping  and  ingot  furnaces,  it  will  be  obvious  that 
additional  energy  is  needed  in  the  former  in  order  to  keep  the  car- 
bide molten.  In  practice,  this  additional  energy  is  less  than  might 
be  supposed,  because  in  such  furnaces  the  carbide  is  often  rendered 
more  easily  fusible  by  the  presence  of ^an  excess  of  lime  in  the  charge. 
It  follows,  therefore,  that  the  carbide  from  running-furnaces  is  fre- 
quently of  a  lower  grade  than  would  be  produced  in  ingot-furnaces. 

In  regard  to  the  size  of  these  furnaces,  it  has  been  found  that  200- 
300  h.p.  is  a  suitable  size  for  an  ingot  furnace.  The  tapping  fur- 
nace can  be  made  decidedly  larger  (up  to  2,000  to  3,000  h.p.), 
being  limited  merely  by  the  size  of  the  electrodes,  which  should  not 
carry  a  current  of  more  than  12-15  amperes  per  square  inch;  larger 
currents  tending  to  overheat  and  "burn"  the  carbide.  In  regard 
to  the  nature  of  the  electrodes,  it  has  been  found  desirable  to  use 
graphite  electrodes  in  ingot  furnaces,  but  in  the  tapping  furnace, 
owing  to  the  excess  of  lime  in  the  charge,  there  is  a  greater  con- 
sumption of  electrode  material,  and  it  is  therefore  necessary  to  use 
the  cheaper  carbon  electrode. 

In  the  production  of  calcium  carbide,  much  carbon  monoxide  is 
liberated.  As  this  gas  liberates  a  large  amount  of  heat  when  burnt, 
it  should  be  utilized  for  burning  the  limestone,  or  preheating  the 
charge.  Owing,  however,  to  the  difficulty  of  suitably  enclosing 
the  furnace  very  little  use  has  so  far  been  made  of  the  carbon  monox- 
ide which  is  generally  allowed  to  burn  above  the  charge  in  the 
furnace. 


RESISTANCE  FURNACES 

Calcium  carbide  is  generally  formed  in  arc- furnaces,  but  it  has  been 
stated  that  in  France  many  carbide  furnaces  are  operated  on  the 
resistance  principle,  that  is  to  say,  without  the  formation  of  an  arc. 
W.  Borchers  in  his  "Electric  Furnaces"  shows  a  furnace  by  Gin  and 
Leleux,  having  one  movable  electrode  which  is  lowered  until  it 
20 


306 


THE  ELECTRIC  FURNACE 


touches  the  molten  carbide,  so  that  no  arc  can  be  produced,  but  the 
heat  would  be  generated  by  the  passage  of  the  electric  current  through 
the  molten  carbide.  On  the  other  hand,  it  has  been  pointed  out1 
that  the  electrodes  should  not  touch  the  molten  carbide,  and  that  an 
arc  must,  therefore,  always  be  present  in  furnaces  of  this  class.  It 
seems  probable  that  when  " Resistance  Furnaces"  are  referred  to, 
the  meaning  is  that  the  furnace  is  operated  with  a  low  voltage  (about 
30  volts) ,  the  electrodes  being  only  slightly  separated  from  the  molten 
carbide.  In  this  connection  it  may  be  pointed  out  that  in  a  furnace 
for  smelting  iron-ores,  where  the  charge  is  moderately  fusible,  the 
current  flows  from  the  electrodes  through  the  melting  charge  to  the 
molten  slag  and  metal  without  the  formation  of  an  arc,  but  that  in  a 
carbide  furnace,  owing  to  the  refractory  character  of  the  charge,  any 


_[]_ *U|jpj  ""-qlfJ  ||  I  III   Ivgj) ^j)   I  II I 


FIG.  125. — Resistance  carbide  furnace. 

separation  between  the  electrodes  and  the  molten  product  entails 
the  formation  of  an  arc.  There  are,  however,  certain  furnaces  which 
undoubtedly  operate  on  the  resistance  principle. 

Tests  on  the  relative  efficiency  of  arc  and  resistance  furnaces  have 
been  made  in  the  laboratory  by  Prof.  Tucker,2  using,  for  a  resistance 
furnace,  a  carbon  rod  heated  by  the  electric  current,  around  which 
the  charge  was  placed.  This  furnace  was  found  to  be  decidedly  more 
economical  than  a  small  arc-furnace,  but  the  conditions  of  the  tests 
did  not  permit  of  a  fair  comparison  between  arc-furnaces  and  resist- 
ance furnaces  in  general.  In  operating  the  resistance  furnace  the 

1  W.  Conrad,  "Electric  Furnaces  for  the  Manufacture  of  Calcium  Carbide 
and  Ferro-silicon."     Electrochem.  and  Met.  Ind.,  vi,  1908,  p.  397. 

2  S.  A.  Tucker  and  others,  "Relative  Efficiency  of  the  Arc  and  Resistance 
Furnace  for  the  Manufacture  of  Calcium  Carbide,"  Trans.  Am.  Electrochem. 
Soc.,  xv,  1909,  p.  411. 


GRAPHITE  AND  CARBIDES  307 

carbon  rod  soon  burned  away,  causing  the  current  to  pass  through 
the  resulting  carbide. 

The  furnace  shown  in  Fig.  125  possesses  many  points  of  interest. 
It  consists  of  a  furnace-hearth,  B  B,  mounted  on  wheels,  and  having 
a  carbon  electrode,  E,  at  each  end;  these  electrodes  forming  part  of 
the  bottom  of  the  furnace.  The  current  passes  between  these  elec- 
trodes through  the  molten  carbide,  so  that  in  this  case  the  heat  is 
developed  entirely  in  the  carbide.  The  portion  of  the  hearth  lying 
between  the  electrodes  is  composed  of  refractory  material,  and  is 
provided  with  passages  for  air-cooling.  This  is  necessary  to  prevent 
the  melting  of  this  part  of  the  hearth.  The  upper  part  of  the  furnace 
consists  of  a  shaft  divided  into  a  number  of  sections,  A  A ,  by  hollow 
walls.  The  gases  produced  from  the  reaction  escape  into  the  hollow 
spaces  in  the  walls,  and  meeting  with  a  supply  of  air,  are  burnt  in 
these  flues  and  heat  the  furnace  before  they  escape. 

The  Production  of  Calcium  Carbide. — The  materials  for  the  pro- 
duction of  calcium  carbide  are  lime,  and  some  form  of  carbon  such 
as  coke,  anthracite  or  charcoal.  It  is  essential  that  they  should 
be  as  pure  as  possible,  and  in  particular,  that  the  lime  should  be  thor- 
oughly burnt  and  quite  free  from  moisture.  It  should  contain  as 
little  magnesia  and  phosphorus  as  possible,  especially  when  it  is 
to  be  used  for  the  production  of  acetylene.  Magnesia  interferes 
with  the  formation  of  calcium  carbide,  as  it  does  not  itself  form  a 
carbide,  and  phosphorus  enters  the  carbide  and  finally  passes  into 
the  acetylene  gas,  as  phosphoretted  hydrogen.  When  this  is  burnt, 
a  haze  is  produced  which  is  very  objectionable.  The  coke  or  other 
fuel  must  also  be  as  free  as  possible  from  ash-forming  ingredients. 

The  proportion  of  lime  and  fuel  employed  is  about  that  required 
by  the  equation,  allowing  of  course  for  the  presence  of  any  impuri- 
ties. The  charge  used  in  the  ingot  furnace  contains,  however, 
a  slight  excess  of  carbon,  and  that  in  the  running  furnace,  a  small 
excess  of  lime.  The  lime  is  roughly  crushed  to  pieces  of  i  in.  in 
size  and  the  coal  is  crushed  to  pea  size.  Finer  crushing,  which  was 
in  use  at  one  time,  leads  to  the  formation  of  a  large  amount  of  dust, 
and  has  been  found  to  be  unnecessary. 

It  is  often  stated  that  i  ton  of  carbide  requires  i  h.p.  year 
for  its  production,  but  the  output  from  a  modern  furnace  is  from  10 
to  13  Ib.  of  commercial  carbide  per  kilowatt  day,  which  is  nearly  2 
tons  per  horse-power  year,  and  in  some  recent  furnaces  as  much  as 
i .  5  tons  of  carbide  have  been  produced  for  every  horse-power 
year  supplied  to  the  plant.  The  cost  of  making  carbide  in  Canada 


308  THE  ELECTRIC  FURNACE 

with  electrical  power  costing  $10  a  horse-power  year  has  been 
stated  to  be  about  $30  per  ton. 

Uses  of  Calcium  Carbide. — Calcium  carbide  is  used  mostly  for 
the  production  of  acetylene  which  is  liberated  when  the  carbide 
reacts  with  water  according  to  the  following  equation: 

CaC2+2H2O  =  C2H2+Ca(HO)2. 

Pure  calcium  carbide  would  yield  366  liters  of  acetylene  per 
kilo  of  carbide,  or  5.86  cu.  ft.  per  pound;  the  gas  being  measured 
at  15°  C.  and  760  mm.  The  commercial  carbide  is  usually  from 
80-85  per  cent,  pure,  and  yields  300  liters  per  kilo  of  carbide,  or 
4.8  cu.  ft.  per  pound.  In  recent  years  another  use  for  the  carbide 
has  been  discovered.  When  calcium  carbide  is  heated  in  a  closed 
retort  to  about  800°  to  1000°  C.  in  the  presence  of  nitrogen,  the 
nitrogen  is  absorbed  with  the  formation  of  calcium  cyanamide, 
graphite  being  liberated  as  shown  in  the  equation: 

CaC2+N2  =  CaCN2+C. 

Calcium  cyanamide  has  a  considerable  use  as  a  fertilizer,  and  can 
also  be  employed  for  the  production  of  ammonia,  by  passing  steam 
over  it  at  a  red  heat,  as  shown  in  the  equation: 

CaCN2+3H2O  =  2NH3+CaCO3. 

The  ammonia  can  be  passed  into  sulphuric  acid  with  the  formation 
of  ammonium  sulphate,  which  is  used  as  a  manure.  In  the  manu- 
facture of  calcium  cyanamide,  the  nitrogen  is  obtained  by  liquifying 
air  and  then  separating  the  oxygen  and  nitrogen  by  fractional  dis- 
tillation, or  by  removing  the  oxygen  from  the  air  by  means  of  copper. 
The  total  output  of  calcium  carbide  in  1909  amounted  to  about 
250,000  tons. 


CHAPTER  XII 

THE  ELECTRIC  SMELTING  OF  ZINC  AND  OTHER  METALS 
ELECTROTHERMIC  PRODUCTION  OF  ZINC 

Although  zinc  is  one  of  the  common  metals,  and  has  long  been 
produced  in  furnaces  fired  by  coal  or  gas,  its  volatility  and  the 
ease  with  which  it  becomes  oxidized  present  serious  difficulties  in 
the  treatment  of  its  ores,  and  many  attempts  have  been  made  to 
overcome  these  difficulties  by  smelting  ores  of  zinc  in  the  electric 
furnace. 

In  the  usual  process  of  zinc  smelting,  the  ores  are  first  roasted, 
to  remove  sulphur  in  the  case  of  sulphide  ores  or  carbonic  acid  in 
the  case  of  carbonate  ores,  and  the  resulting  oxide  of  zinc  is  mixed 
with  about  one-half  its  weight  of  coal  and  heated  in  retorts  or  muffles 
made  of  fire-clay.  In  order  to  complete  the  reduction  of  the  oxide 
to  the  metallic  state  it  must  be  heated  to  a  temperature  above  the 
boiling-point  of  the  zinc,  which  is  consequently  given  off  as  vapor, 
passing  in  that  form  out  of  the  retort,  and  is  condensed  to  the  liquid 
metal  in  a  condenser,  from  which  it  can  be  removed  and  poured 
into  molds. 

The  residue  is  then  removed  from  the  retort  and  the  operation  re- 
peated. The  retorts  are  heated  externally  by  coal  or  gas  firing,  and 
as  the  ore  must  be  heated  to  about  1,200°  C.,  or  2,200°  F.,  the  retorts 
cannot  usually  be  made  very  large  and  frequently  only  hold  about 
100  Ib.  of  the  ore  mixture;  from  which  it  will  be  seen  that  the 
cost  of  labor  in  zinc  smelting  is  likely  to  be  high.  The  utilization  of 
heat  in  these  furnaces  is  also  very  poor  on  account  of  its  slow  trans- 
mission through  the  walls  of  the  retorts;  the  heat  efficiency  of  such  a 
furnace  being  given  by  Prof.  Richards1  as  under  7  per  cent.  At  the 
high  temperature  of  zinc  distillation  the  retorts  only  last  about  a 
month,  and  their  renewal  forms  a  considerable  item  of  expense. 
Other  difficulties  are  met  in  the  condensation  of  the  zinc  vapor,  as 
this  does  not  all  collect  in  the  liquid  state,  but  in  part  as  a  powder, 
which  cannot  be  melted  together,  while  a  part  of  the  vapor  escapes 
altogether. 

1  J.  W.  Richards'  Metallurgical  Calculations,  Part  I,  p.  80. 

309 


310  THE  ELECTRIC  FURNACE 

Most  of  the  difficulties  that  have  been  referred  to  are  caused  by  the 
necessity  of  heating  the  ore  in  a  number  of  small  retorts,  heated  ex- 
ternally, instead  of  in  a  large  furnace  in  which  the  heat  could  be  pro- 
duced in  close  contact  with  the  ore.  Attempts  have  been  made  to 
reduce  the  ore  in  some  form  of  blast-furnace,  but  the  zinc  was  too 
easily  oxidized  by  the  furnace  gases,  and  it  was  not  possible  to  con- 
dense the  zinc  to  the  liquid  state.  Zinc  oxide  suitable  for  making 
paint  is,  however,  produced  in  small  blast-furnaces,  and  is  filtered  out 
of  the  furnace-gases  by  passing  them  through  woolen  bags. 

In  the  electric  furnace,  heat  can  be  produced  without  the  necessity 
of  blowing  air  into  the  charge;  the  atmosphere  in  the  furnace  can  be 
made  thoroughly  reducing,  so  that  no  zinc  will  be  oxidized,  and  the 
gases  leaving  the  furnace  are  no  more  than  leave  the  zinc  retort  in  the 
usual  process,  so  that  the  condensation  of  the  zinc  should  be  satis- 
factory. The  production  of  heat  electrically,  in  the  midst  of  the  ore 
mixture,  enables  the  furnace  to  be  made  of  any  convenient  size,  and 


FIG.  126. — Cowles  zinc  furnace. 

thus  greatly  reduces  the  expense  of  labor,  while,  as  the  heat  has  not 
to  be  transmitted  through  the  furnace  walls,  these  will  be  far  more 
permanent  and  a  great  source  of  expense  will  thus  be  avoided. 

Although  the  advantages  that  could  be  gained  by  smelting  zinc- 
ores  electrically  were  very  obvious,  the  practical  application  of  elec- 
trical heating  to  this  process  has  not  been  easy.  The  first  electrical 
furnace  for  distilling  zinc-ores  was  patented  by  the  Cowles  brothers 
in  1885,  and  consisted,  Fig.  126,  of  a  fire-clay  tube,  A,  closed  at  one 
end  by  a  carbon  plug,  B,  and  at  the  other  end  by  a  carbon  crucible, 
C,  and  lid,  D.  The  charge  of  roasted  ore  and  coal  was  contained  in 
the  tube,  and  electrical  connections  were  made  to  the  carbon  plate 
and  crucible  so  that  an  electric  current  flowed  through  and  heated  the 
ore  in  the  tube.  The  tube  was  surrounded  with  some  suitable 
material  to  reduce  the  loss  of  heat.  The  vaporized  zinc  and  other 
gaseous  products  of  the  process  escaped  through  a  hole  into  the  cruci- 
ble, where  the  zinc  condensed  to  a  liquid  at  Z,  while  the  remaining 
gases  passed  away  by  the  pipe  E.  The  furnace  was  practically  an 


THE  ELECTRIC  SMELTING  OF  ZINC 


311 


electrically  heated  zinc  retort,  and,  as  shown  in  the  figure,  the  process 
was  intended  to  be  intermittent  in  action,  one  charge  being  ex- 
hausted and  then  discharged  before  another  could  be  introduced. 
Provision  could,  however,  have  been  made  for  the  continuous  charg- 
ing and  discharging  of  such  a  furnace,  but  the  process  was  never 
completed. 

A  furnace  patented  in  1904  by  W.  Me  A.  Johnson,1  of  the  Lanyon 
Zinc  Co.,  Fig.  127,  is  practically  the  same  as  the  Cowles'  furnace,  but 
it  is  designed  on  a  larger  scale,  and  care  has  been  taken  to  prevent 
the  overheating  of  the  walls  of  the  furnace.  It  consists  of  an  arched 
chamber,  A ,  with  end  walls,  B  and  C,  and  a  flue,  D,  through  which  the 
zinc  and  other  gaseous  products  of  the  operation  can  pass.  The 
whole  furnace  is  supported  upon  I-beams,  thus  enabling  the  air  to 
pass  underneath  and  prevent  overheating.  The  furnace  is  con- 
structed of  fire-clay  bricks,  but  as  additional  protection,  a  layer,  M , 


FIG.  127. — Johnson  zinc  furnace. 

of  refractory  material,  such  as  silica,  high-grade  fire  clay  or  bauxite,  is 
placed  on  the  hearth.  The  ore  mixture  consists  of  roasted  ore  mixed 
with  enough  coke  to  reduce  the  zinc  and  to  carry  the  electric  current. 
Some  of  the  ore  to  be  treated  contains  considerable  amounts  of  iron, 
lime,  lead  and  copper,  and  would  be  likely  to  flux  the  walls  of  the 
furnace.  This  low-grade  ore  is,  therefore,  placed  in  the  middle  and 
upper  part  of  the  furnace  at  K,  being  separated  from  the  floor  and 
walls  by  a  layer  of  purer  ore,  /.  All  the  ore  is  mixed  with  enough  coke 
to  reduce  the  zinc  it  contains,  but  in  order  to  prevent  the  overheating 
of  the  walls  care  is  taken  that  the  mixture  K  shallbe  a  better  electrical 
conductor  than  the  mixture  /,  so  that  the  current  will  pass  mainly 
through  the  middle  of  the  furnace.  E  and  F  are  heaps  of  coke  serving 
as  electrodes,  the  current  flowing  between  E  and  F  through  the  ore 

1  W.  McA.  Johnson,  Electric  Zinc  Furnace,  U.  S.  patent  814,050,  filed  May  24, 
1904,  Electrochemical  Industry,  vol.  iv,  p.  152. 


312  TEE  ELECTRIC  FURNACE 

mixture.  Electrical  .contact  is  made  with  the  coke  by  means  of 
graphite  or  carbon  blocks  and  rods  passing  through  the  front  and  back 
of  the  furnace.  G  and  H  are  connected  to  one  cable  bringing  the 
current,  and  make  contact  with  the  coke  F,  while  two  similar  termi- 
nals connect  the  other  cable  to  the  coke  E.  This  furnace  is  the  same 
in  principle  as  an  ordinary  zinc  retort,  but  the  production  of  the  nec- 
essary heat  within  the  retort,  which  can  only  be  effected  electrically, 
enables  the  dimensions  of  the  retort  to  be  increased  to  any  desirable 
extent,  and  the  walls,  instead  of  being  thin,  as  was  necessary  when 
the  heat  had  to  pass  through  them,  can  be  made  of  any  suitable  thick- 
ness. The  furnace  is  necessarily  intermittent  in  action,  and  would  be 
allowed  to  cool  somewhat  to  allow  of  the  spent  ore  being  removed 
through  some  convenient  opening  and  a  fresh  charge  being  carefully 
arranged  in  the  furnace  before  it  could  again  be  heated.  The  zinc 
vapors  passing  through  the  flue  D  would  enter  a  system  of  condensing 
chambers,  the  first  section  of  which  would  be  kept  at  a  temperature 
above  the  melting-point  of  zinc  in  order  to  obtain  that  metal  in  the 
molten  condition. 

The  electric  zinc  furnace  of  C.  P.  G.  de  Laval,  Fig.  128,  was 
patented  in  1903,  and  has  this  advantage  over  the  Johnson  furnace, 
that  the  ore  mixture  can  be  continually  charged  into  the  furnace, 
and  that  the  residues  are  fused  and  can  be  tapped  out  at  intervals 
without  interrupting  the  operation  of  the  furnace.  The  heating 
is  effected  by  an  arc  which  is  maintained  between  two  carbon 
electrodes,  one  of  which  is  shown  at  E.  The  ore  mixture  is  intro- 
duced continuously  by  means  of  a  charging  shaft,  A ,  or  by  a  hopper 
and  screw  feed  through  the  wall,  F,1  and  forms  a  heap,  C,  in  the 
furnace,  where  it  is  gradually  heated,  the  zinc  reduced  to  the  metallic 
state  and  distilled,  and  the  residues  finally  melted  by  the  heat  of 
the  arc.  The  vaporized  zinc  and  the  gases  produced  by  the  action 
of  the  coal  on  the  ore,  escape  by  a  passage,  D,  to  condensing  chambers. 
The  heaping  up  of  the  ore  in  the  furnace  serves  to  protect  the  charg- 
ing aperture  and  the  gradual  heating  of  the  ore  is  probably  an  impor- 
tant feature  of  the  process,  as  it  allows  the  zinc  oxide  to  be  reduced 
to  the  metallic  state,  and  the  resulting  zinc  to  escape  from  the  ore 
before  fusion  sets  in,  as  it  is  difficult  to  liberate  the  metal  from  its 
ore  when  in  a  pasty  or  fused  state.  The  utilization  of  the  electrical 
heat  in  this  furnace  is  not  perfect,  but  the  operation  is  simple  and 
therefore  not  likely  to  give  trouble.  The  process  has  been  in 

1  C.  G.  P.  De  Laval,  U.  S.  patent  768,054,  Electrochemical  Industry,  vol.  ii, 
p.  423. 


THE  ELECTRIC  SMELTING  OF  ZINC 


313 


commercial  use  for  several  years  in  Europe.  In  the  year  1906, 
3,000  h.p.  was  employed  at  Trollhattan  (Sweden)  in  the  reduction 
of  ore  and  zinc  ashes  (galvanizers'  waste),  4,000  h.p.  at  Sarpsborg 
(Norway)  in  the  reduction  of  zinc  ashes,  and  1,800  h.p.  at  Hallsta- 
hammar  in  the  smelting  of  ore.1 

De  Laval,  in  common  with  all  others  who  have  smelted  zinc  ores 
electrically,  found  that  the  zinc  did  not  condense  satisfactorily  to 
the  molten  state,  but  remained  largely  in  the  form  of  a  powder 
which  could  not  be  melted  together.  This  difficulty  was  most 
pronounced  in  the  smelting  of  ores,  and  for  some  years  the  process 
was  largely  confined  to  the  distillation  of  zinc  dross  and  other  metallic 


FIG.  128. — Laval  zinc  furnace. 


products.  The  works  at  Trollhattan  and  Sarpsborg  were  studied 
by  F.  W.  Harbord  in  1911,  and  his  report,2  from  which  the  following 
particulars  are  taken,  shows  that  some  progress  has  been  made  in  the 
electrothermic  smelting  of  zinc  ores. 

At  the  Trollhattan  works  the  arc-furnace,  Fig.  128,  has  been 
replaced  by  a  "resistance"  furnace,  similar  in  principle  to  the 
Salgues  furnace,  Fig.  129;  but  some  of  the  arc- furnaces  were  still  in 
operation  at  the  Sarpsborg  works.  The  arc-furnace  gives  as  good 
results  as  the  resistance  furnace,  but  consumes  70  per  cent,  more 

1  Report  of  the  Commission  to  investigate  the  zinc  resources  of  British  Colum- 
bia and  the  conditions  affecting  their  exploitation,  Ottawa,  1906. 

2  Zinc  Smelting  at  Trollhattan,  Eng.  and  Min.  Journal,  Feb.  10,  1912,  p.  314. 


314  THE  ELECTRIC  FURNACE 

energy  for  an  equal  output.  This  difference  is  easily  explained  by 
the  inefficient  transfer  of  heat  from  the  arc  to  the  heap  of  ore. 

The  "resistance"  furnaces  have  two  electrodes  as  in  Fig.  129, 
the  lower  one  being  a  carbon  block  bedded  in  the  bottom  of  the 
furnace,  and  the  upper,  movable  electrode  being  a  vertical  carbon 
rod  weighing  1,500  lb.,  having  a  cross-section  of  336  sq.  in.,  and  a 
length  of  10  ft. 

Each  furnace  uses  350  electrical  horse-power,  which  is  supplied 
at  about  100  volts;  corresponding  to  a  current  of  about  2,600  amperes, 
or  8  amperes  for  each  square  inch  of  electrode-section.  With  100 
volts  it  is  quite  likely  that  an  arc  may  be  formed  between  the  elec- 
trodes and  the  slag,  but  any  arc  will  be  surrounded  by  the  charge, 
and  the  heat  will  be  utilized  far  more  perfectly  than  in  the  original 
Laval  furnace.  The  furnace  holds  about  3  tons  of  charge  and 
smelts  2.8  metric  tons  of  ore  per  24  hours. 

The  smelting  process  is  carried  out  in  two  stages;  in  the  first,  the 
ore  is  smelted  with  anthracite  or  coke  and  suitable  fluxes,  yielding 
some  molten  zinc  and  a  large  amount  of  "blue  powder"  and  oxide. 
The  powder  contains  about  56  per  cent,  of  zinc  and  20  per  cent,  of 
lead.  It  is  recharged  in  another  furnace  with  an  admixture  of  ore 
and  flux,  and  yields  a  larger  proportion  of  molten  zinc  than  was 
obtained  from  the  ore  furnace. 

In  Mr.  Harbord's  test,  three  furnaces  were  smelting  an  ore  charge 
of  300  kg.  Broken  Hill  slime  (roasted),  10  kg.  calamine,  and  75  kg. 
of  coke-dust;  four  furnaces  were  retreating  the  powder  (from  all  the 
furnaces),  using  100  kg.  Broken  Hill  slime  (roasted),  200  kg.  powder, 
25  kg.  coke-dust  and  5  kg.  lime.  The  relative  yield  of  zinc  and  blue 
powder  from  the  ore  furnaces  and  the  powder  furnaces  is  not  stated, 
but  from  the  figures  quoted,  it  will  be  seen  that  on  the  average  all  the 
zinc  obtained  in  the  molten  state  has  been  smelted  twice  before  it 
reaches  that  condition.  The  whole  consumption  of  electrical  energy 
and  of  electrodes,  both  for  the  ore  smelting  and  the  redistillation  of 
the  blue  powder,  amounted  to  2,078  kw.-hours  and  31.5  kg.  of  elec- 
trodes per  metric  ton  of  ore  smelted. 

Although  the  test  was  on  a  considerable  scale,  lasting  27  1/2  days, 
and  treating  537  tons  of  ore,  the  recovery  of  metals  was  low,  being 
73  per  cent,  of  the  zinc,  79  per  cent,  of  the  lead,  and  50  per  cent,  of 
the  silver  in  the  ore.  Part  of  the  loss  was  caused  by  the  metals  soak- 
ing into  the  brickwork  of  the  furnace,  and  part  as  fume  which  was 
not  completely  recovered.  The  Broken  Hill  slime  contained  about 
34  per  cent,  of  zinc,  24  per  cent,  of  lead,  and  30  oz.  of  silver  per  ton. 


THE  ELECTRIC  SMELTING  OF  ZINC 


315 


The  ore  is  charged  through  the  roof  of  these  furnaces,  but  in  a 
later  type,  a  continuous  side-feed  has  been  introduced,  and  is  expected 
to  give  better  results.  A  mechanical  stirrer  has  also  been  used  for 
separating  the  molten  zinc  from  the  powder. 

M.  A.  Salgues1  wrote,  in  the  year  1903,  an  account  of  the  electro- 
metallurgy of  zinc,  and  figured  two  or  three  furnaces,  one  of  which, 
intended  for  use  with  100  kilowatts,  is  illustrated  in  Fig.  129.  It 
consists  of  a  chamber,  built  in  two  parts,  A  and  B,  to  facilitate  clean- 


FIG.  129. — Salgues  zinc  furnace. 

ing  and  repairing,  an  off-take,  C,  for  the  passage  of  the  zinc  and  other 
gases,  a  tap-hole,  D,  and  two  charging  and  poking  holes,  one  of  which 
is  shown  at  E.  Heat  is  produced  in  the  charge  of  ore  by  the  passage 
of  an  electric  current  between  the  carbon  electrodes,  F  and  G;  F  being 
movable  and  supported  by  a  suitable  electrode  holder,  while  G  is 
set  in  the  base  of  the  furnace,  and  electrical  contact  is  made  with  it 
by  the  bar  of  metal,  H.  The  furnace  is  built  of  fire-bricks  inside  an 
iron  jacket  which  is  cooled  by  sprinklers  shown  at  K,  the  lower  car- 
1  Salgues,  Bull.  Soc.  Ing.,  civ,  1903,  p.  174. 


316  THE  ELECTRIC  FURNACE 

bon  holder  having  a  special  water-cooling  device,  shown  at  L.  The 
hearth  of  the  furnace  is  lined  with  sand,  /,  as  is  common  in  many 
smelting  furnaces. 

Salgues  draws  special  attention  to  the  means  by  which  he  keeps 
the  furnace  air-tight  around  the  upper  electrode  and  at  the  poking 
and  charging  holes.  For  this  purpose  he  provides  a  heavy  cast-iron 
plate,  M,  in  which  are  holes  for  the  electrode  and  for  charging,  the 
latter  being  closed  by  lids,  O.  The  gases  in  the  furnace,  being  under 
a  slight  pressure,  rush  out  through  any  opening,  such  as  the  crack 
around  the  electrode,  but  a  ring  of  asbestos,  N,  delays  the  gases 
a  little,  and  the  zinc  vapor  will  then  condense  on  the  iron  plate  (which 
is  cooled  by  a  jet  of  water),  and  immediately  closes  the  crack.  In 
the  same  way,  after  charging  or  poking,  the  crack  between  the  lid,  0, 
and  its  seat  is  immediately  sealed  by  the  zinc,  which  condenses  there. 

The  charge,  consisting  of  roasted  ore  and  the  necessary  carbon  for 
its  reduction,  being  introduced  at  £,  from  time  to  time,  lies  around 
the  electrode,  F,  and  as  it  becomes  heated  the  zinc  is  reduced  and 
volatilized,  passing  through  C  to  condensing  chambers;  while  the 
residue  of  the  ore,  which  would  need  to  be  fusible,  collects  in  the 
molten  state  at  R,  and  is  tapped  out  at  intervals.  This  furnace  is 
continuous  in  operation,  and  incidentally  allows  of  the  smelting  of 
associated  metals,  such  as  lead,  which  will  collect  in  the  furnace  and 
be  tapped  out  with  the  slag.  The  heat  may  be  produced  in  this  fur- 
nace by  the  passage  of  the  current  through  the  molten  slag,  R, 
but  if  the  electrode,  F,  were  raised  higher  an  arc  would  be  pro- 
duced. Salgues  experimented  at  Champagne  (Ariege),  France,  with 
a  modified  carbide  furnace  of  100  kilowatts,  and  using  ores  carry- 
ing 40  to  45  per  cent,  of  zinc,  fed  cold  into  the  furnace,  he  obtained 
a  yield  of  5  kg.  of  zinc  per  kilowatt-day. 

It  is  in  the  smelting  of  mixed  ores  containing  both  zinc  and 
lead,  usually  associated  with  silver,  that  the  greatest  advantage  of 
electrical  smelting  may  be  expected.  Such  ores  are  very  difficult  to 
treat  by  ordinary  furnace  methods,  because,  if  smelted  as  a  lead  ore 
in  the  blast-furnace,  the  zinc  makes  infusible  slags  and  chokes  up 
the  furnace  with  deposit?  of  fume,  and  none  of  the  zinc  is  recovered. 
If  treated  as  a  zinc  ore,  the  lead  makes  the  ore  fusible  so  that  it 
corrodes  the  retorts,  beside  yielding  an  impure  zinc  containing  some 
lead.  When  treated  as  a  zinc  ore,  the  lead  and  silver  can  be  recov- 
ered by  smelting  the  residues  from  the  zinc  retorts. 

The  Broken  Hill  ores  are  notable  examples  of  a  mixed  sulphide 
of  lead  and  zinc  which  cannot  be  separated  at  all  completely  by 


THE  ELECTRIC  SMELTING  OF  ZINC  317 

mechanical  means  and  must  be  treated  as  a  whole  by  smelting  or 
chemical  methods.  It  occurred  to  the  writer,  about  the  year  1900, 
that  such  ores  could  be  smelted  electrically  so  as  to  recover  at  one 
operation  the  zinc,  lead  and  silver  from  the  ore;  the  zinc  being  dis- 
tilled and  condensed,  while  the  lead  carrying  the  silver  from  the  ore 
would  collect  in  the  molten  condition  as  in  ordinary  blast-furnace 
practice.  Numerous  experiments  on  a  laboratory  scale  showed  that 
this  could  be  accomplished,  and  that  the  extraction  of  the  lead  was 
particularly  good,  only  traces  of  that  metal  remaining  in  the  slag. 

The  furnace  in  which  some  of  these  experiments  were  made  is 
shown  in  Fig.  130. l  It  consisted  of  a  rectangular  chamber,  AB,  in 
which  the  ore  was  smelted,  a  charging  shaft,  C,  for  introducing  the  ore, 
and  chambers,  F  to  K,  for  condensing  and  collecting  the  zinc.  The 
electric  current  was  introduced  by  means  of  the  carbon  electrodes, 
D  and  E,  which  dipped  into  the  molten  slag  in  the  furnace.  In 
starting,  a  quantity  of  slag  was  melted  and  poured  into  the  furnace 
which  had  previously  been  heated.  The  electrodes  were  then  intro- 
duced, and  the  current  switched  on.  The  mixture  of  roasted  ore,  car- 
bon and  fluxes  was  poured  into  the  shaft,  C,  which  was  kept  nearly 
full  during  the  operation  of  the  furnace.  The  furnace  was  essentially 
a  resistance  furnace,  the  heating  being  accomplished  by  the  passage  of 
the  current  through  the  molten  slag,  and  the  furnace  was  made  long 
and  narrow  in  order  that  the  electrical  resistance  might  not  be  too  low. 
Occasionally,  however,  the  electrodes  would  become  too  short  to 
reach  the  slag,  and  then  an  arc  was  formed.  The  products  of  the 
operation  were  lead,  which  collected  at  L,  and  was  tapped  out  by  the 
spout,  R,  slag  which  is  shown  at  S,  and  was  tapped  out  by  the  spout, 
r,  and  zinc  vapor  and  other  gases  which  left  the  furnace  by  the  open- 
ings FF.  The  condensing  system  consisted  of  the  chamber,  F,  in 
which  a  small  amount  of  molten  zinc  collected,  and  an  extensive 
system  of  iron  pipes,  GH,  and  JK,  in  which  the  greater  part  of  the 
zinc  collected  in  the  form  of  zinc  powder.  The  gases  escaping  with 
the  zinc  vapor  were  mostly  carbon  monoxide  which  burned  at  the 
end  of  the  condensing  system. 

The  cost  of  smelting  sulphide  ores  of  zinc,  by  the  usual  process,  is 
materially  increased  by  the  necessity  of  a  very  complete  roasting 
operation  before  the  distillation  of  the  zinc.  Any  sulphur  left  in  the 

1  This  furnace  was  devised  by  the  author  and  Mr.  L.  B.  Reynold  ,  who  did 
most  of  the  experimental  work.  They  have  obtained  the  following  patents  on 
the  electrical  smelting  of  lead-zinc  ores: — Canadian  102,311,  Australian  1,681, 
German  183,470,  Mexican  4,710. 


318 


THE  ELECTRIC  FURNACE 


FIG.  130. — Experimental  zinc  furnace. 


THE  ELECTRIC  SMELTING  OF  ZINC  319 

ore  holds,  as  a  rule,  about  twice  its  weight  of  zinc  in  the  residues,  and 
it  is,  therefore,  the  practice  to  leave  no  more  than  about  i  per  cent, 
of  sulphur  in  the  roasted  ore.  So  complete  a  removal  of  sulphur  in- 
volves a  prolonged  roasting  at  a  very  high  temperature,  thus  largely 
increasing  the  cost  as  well  as  the  loss  by  volatilization  of  the  lead  and 
silver  in  the  ore.  Some  inventors  have  tried  to  avoid  this  by  smelt- 
ing the  ore,  unroasted,  in  the  electric  furnace,  with  the  addition  of 
some  reagent  for  absorbing  the  sulphur;  iron,  iron-ore,  alkaline  salts 
and  lime  being  suitable  for  this  purpose. 

F.  T.  Snyder  patented1  a  process  for  obtaining  zinc  from  a  sulphide 
ore  of  this  metal  without  roasting.  The  ore  is  mixed  with  carbon  and 
fluxes  (iron  and  lime),  and  smelted  upon  a  bath  of  molten  slag  in  an 
electric  furnace  from  which  the  air  is  excluded.  The  inventor  claims 
that  the  carbon  reacts  with  the  sulphur  of  the  ore  and  forms  carbon 
bisulphide,  which  is  volatilized,  liberating  the  zinc.  It  is  not  stated 
whether  the  iron  and  lime  used  as  fluxes  played  any  part  in  absorb- 
ing the  sulphur  and  liberating  the  zinc.  Direct  current  is  used,  and 
it  is  stated  that  some  electrolytic  effect  is  produced;  the  zinc  being 
liberated  at  one  electrode  and  the  carbon  bisulphide  at  the  other 
electrode.  In  one  experiment,  ore  containing  20  per  cent,  zinc,  20 
per  cent,  iron,  5  per  cent,  lead,  35  per  cent,  sulphur,  and  20  per  cent. 
of  silica  and  alumina  was  mixed  with  iron  and  lime  (and  carbon)  and 
fed  into  an  electric  furnace  provided  with  carbon  electrodes,  between 
which  scrap  lead  had  been  placed  for  starting  the  furnace.  A  direct 
current  of  1,500  to  1,800  amperes  at  7  to  15  volts  was  employed, 
heating  the  furnace  to  about  1,200°  C.  The  ore  melted  and  was  re- 
duced, zinc  being  liberated  in  the  form  of  vapor  near  one  electrode, 
while  carbon  bisulphide  was  formed  near  the  other  electrode.  It  is 
claimed  that  at  least  94  per  cent,  of  the  zinc  in  the  charge  can  be  re- 
covered by  this  process. 

Mr.  Snyder  has  experimented  with  induction  furnaces  for  smelting 
zinc  and  other  ores.  The  furnace  represented  in  Fig.  13 12  being 
intended  for  ores  in  general,  while  a  zinc-smelting  furnace3  would  be 
provided  with  chambers  for  condensing  the  zinc.  The  furnace 
shown  in  Fig.  131,  which  could  be  used  for  smelting  lead  ores,  has  a 

•  l  F.  T.  Snyder,  U.  S.  patent  814,810,  filed  June  23,  1905,  Electrochemical 
Industry,  vol.  iv,  p.  152. 

2  Induction  furnace,  F.  T.  Snyder,  U.  S.  patent  825,359.     Application  filed 
July  15,  1904.     See  Electrochemical  Industry,  vol.  iv,  p.  319. 

3  Induction  Furnace  for  Zinc,  F.  T.  Snyder,  U.  S.  patent  859,134.     See  Electro- 
chemical Industry,  vol.  v,  p.  323. 


320 


THE  ELECTRIC  FURNACE 


THE  ELECTRIC  SMELTING  OF  ZINC 


321 


laminated  iron  core,  CC,  a  pair  of  primary  coils,  PP,  and  a  secondary 
circuit  made  up  of  the  molten  slag,  S,  the  molten  metal,  MM,  and 
bars,  Nj  of  copper  or  some  other  metal  which  should  not  be  attacked 
by  the  molten  metal,  M.  The  alternating  current  in  PP,  supplied  by 
the  generator,  D,  causes  a  much  larger  low- voltage  current  to  flow 
around  the  secondary  circuit,  N  M  S  M  N,  composed  of  the  copper 
bars,  N,  the  molten  metal,  M,  and  the  molten  slag,  S.  As  the  slag 
has  a  higher  electrical  resistance  than  the  other  parts  of  this 
circuit,  the  greater  part  of  the  heat  will  be  developed  in  it,  and  the 
ore  introduced  through  the  hoppers  will  be  heated,  reduced  and 
melted  by  contact  with  the  superheated  slag.  The  iron  core, 


FIG.  132. — Snyder  furnace  for  obtaining  liquid  zinc. 

CC,  passes  through  the  middle  of  the  furnace,  and  is  therefore  pro- 
vided with  water-cooling  devices,  not  shown  in  the  drawing,  to  avoid 
overheating. 

Another  zinc  furnace  invented  by  Mr.  Snyder1  is  shown  in  Fig.  132. 
It  is  designed  for  the  treatment  of  lead-zinc  ores,  with  the  special 
intention  of  obtaining  the  resulting  zinc  in  a  coherent  liquid  state, 
instead  of  in  the  form  of  a  powder.  The  furnace  is  constructed  on 
the  lines  of  a  lead  blast-furnace,  having  a  water- jacketed  smelting 

1  Drawing  and  description  sent  to  the  author  by  Mr.  Snyder.     Compare  F.  T. 
Snyder,  U.  S.  patent  859,133.     See  Electrochemical  Industry,  vol.  v,  p.  323. 
21 


322  THE  ELECTRIC  FURNACE 

shaft,  bb,  and  a  crucible  aa,  holding  the  molten  lead  and  molten  slag 
produced  in  the  operation.  A  siphon  tap,  O,  enables  the  molten 
lead  to  flow  out  of  the  furnace,  and  the  slag  and  matte  formed  are 
tapped  out  through  the  hole,  /.  The  special  feature  for  obtaining 
liquid  zinc  is  the  provision  of  the  water-jackets,  bb}  and  of  channels, 
gg,  at  the  bottom  of  the  water-jackets.  The  charge  contains  partly 
roasted  ore,  carbon  and  fluxes,  and  as  it  descends  in  the  furnace, 
the  zinc  and  other  metals  are  reduced  to  the  metallic  state,  and  gases 
such  as  carbon  monoxide  are  liberated.  A  part  of  these  gases  is 
liberated  in  the  upper  and  cooler  part  of  the  shaft,  so  that  the  zinc 
vapor,  which  is  not  formed  until  the  ore  reaches  the  hotter  part  near 
the  bottom  of  the  shaft,  is  less  diluted  by  permanent  gases  than  it 
would  be  if  the  zinc  and  the  gases  were  all  liberated  in  a  common 
chamber  as  would,  be  the  case  in  the  furnace  of  Fig.  131.  The  zinc 
vapor  passes  up  the  shaft  with  the  other  gases,  but  on  reaching  the 
cooler  parts  of  the  ore,  it  is  largely  condensed  and  passes  down  again 
with  the  descending  ore  to  the  hotter  parts  of  the  furnace.  The 
result  of  this  process  is  that  the  zinc  vapor  becomes  concentrated  in 
the  lower  part  of  the  furnace,  and  finally  begins  to  condense  in  the 
liquid  state  in  the  vicinity  of  the  water-cooled  walls,  bb.  The  con- 
densation of  the  zinc  vapor  occurs  more  freely  at  the  sides  of  the 
furnace  than  at  the  ends,  since  the  former  are  further  from  the  carbon 
electrodes,  and  are  therefore  cooler.  The  condensed  zinc  was  in- 
tended to  flow  out  of  the  furnace  through  the  channels,  gg,  beneath 
the  sides  of  the  furnace,  and  to  collect  at  hh.  The  molten  materials 
in  the  bottom  of  the  furnace  are  lead,  L,  matte,  M,  and  slag,  S.  The 
slag  becomes  congealed  around  the  sides  and  ends  of  the  furnace,  and 
in  this  way  it  was  expected  to  maintain  the  channel  through  which  the 
zinc  flows.  The  solidified  slag  also  serves  to  prevent  any  leakage  of 
the  current  into  the  metal  of  the  water-jacket.  Such  leakage  could 
take  place,  however,  higher  up  in  the  furnace  where  there  is  no  slag 
to  form  a  crust  on  the  iron. 

The  furnace  is  operated  by  three-phase  current  which  is  supplied 
by  three  transformers.  The  secondary  windings  of  these  are  con- 
nected in  star  grouping,  one  terminal  of  each  being  connected  to 
one  of  the  three  electrodes,  and  the  other  terminal  to  a  common 
conductor  leading  to  the  bottom  of  the  furnace. 

In  smelting  a  sulphide  ore  of  lead  and  zinc  in  this  furnace,  it  is 
first  roasted  until  the  sulphur  is  reduced  to  about  8  per  cent,  and 
then  smelted  in  admixture  with  coke  or  charcoal,  and  fluxes.  The 
charge  is  proportioned  so  that  the  resulting  slag  will  be  high  in 


THE  ELECTRIC  SMELTING  OF  ZINC  323 

lime  and  silica  (at  least  50  per  cent,  of  the  latter),  as  such  a  slag, 
on  account  of  its  high  melting  temperature,  will  not  retain  any  con- 
siderable quantity  of  zinc  and  will  have  a  high  electrical  resistance. 
A  considerable  amount  of  matte  is  preferred,  enough  iron  being 
present  in  the  charge  to  prevent  much  of  the  zinc  entering  the  matte. 

With  regard  to  the  amount  of  electrical  energy  required  to  smelt 
a  ton  of  roasted  zinc  ore  the  following  data  may  be  given.  Salgues 
states  that  from  a  40  or  45  per  cent,  ore  he  extracted  5  kg.  of  zinc 
per  kilowatt-day.  This  would  correspond  to  1,660  kw.-hours 
per  2,000  Ib.  of  zinc  ore  if  the  zinc  obtained  amounted  to  38  per 
cent,  of  the  ore.  Casaretti  and  Bertani1  produced  at  Bergamo, 
Italy,  9  kg.  of  zinc  per  kilowatt-day,  which,  on  a  zinc  extraction  of 
38  per  cent,  of  the  ore,  would  mean  920  kw.-hours  per  2,000  Ib.  of 
ore.  The  author,  using  a  mixed  lead-zinc  ore  carrying  about 
25  per  cent,  of  each  metal,  was  able  to  extract  both  metals  with  an 
expenditure  of  from  800  to  850  kw.-hours  per  2,000  Ib.  of  ore,  using 
the  ore  cold,  and  in  a  small  furnace  of  only  15  kw. 

In  this  test  the  furnace  had  been  charged  with  molten  slag,  and 
heated  to  the  working  temperature,  before  the  introduction  of  the 
ore-charge.  The  power  measurements  being  made  during  the  time 
needed  to  distil  the  zinc  and  smelt  the  residue.  The  zinc  was 
obtained  almost  entirely  in  the  form  of  powder. 

Snyder2  has  made  a  calculation  based  partly  on  the  fuel  needed  in 
blast-furnace  lead-smelting  and  partly .  on  the  heat  theoretically 
needed  to  reduce  and  distil  zinc  from  its  ores,  and  gives  the  formula, 
623  +  5.4  times  the  percentage  of  zinc,  for  the  kilowatt-hours  needed 
per  2,000  Ib.  of  a  lead-zinc  ore. 

A  more  recent  calculation  by  G.  Gin,3  corrected  by  J.  W.  Richards,4 
gives  the  amount  of  electrical  energy  needed  for  smelting  2,204 
Ib.  of  a  50  per  cent,  zinc  ore  as  1,530  kw.-hours,  or  about  1,400  per 
short  ton.  The  ore  was  a  calcined  calamine  containing: 

ZnO 40 . 50  per  cent.  1 

„  0.~  > Zn,  50. 05  per  cent. 

ZnSiO3 38.07  per  cent.  J 

Fe2O3 9 . 60  per  cent. 

AloSiOs 8. 10  per  cent. 

CaO 2 . 80  per  cent. 

1  Casaretti  and  Bertani,  Report  of  Commission  on  zinc  resources  of  British 
Columbia,  1906,  p.  131. 

2  Snyder,  Jour.  Can.  Min.  Inst.,  vol.  viii,  1905,  p.  130. 

3  G.  Gin,  The  Electrometallurgy  of   Zinc.     Trans.  Am.  Electrochem.    Soc., 
1907,  vol.  xii,  p.  117. 

4  J.  W.  Richards,  Metallurgical  Calculations,  vol.  iii,  p.  611. 


324  THE  ELECTRIC  FURNACE 

Additional  lime  was  added  to  flux  the  silica,  and  carbon  equal  to 
twice  the  theoretical  requirement.  The  gases  and  zinc-vapor  are 
all  supposed  to  leave  the  furnace  at  1,200°  C.,  and  a  sufficient  allow- 
ance has  been  made  for  the  loss  of  heat  by  radiation  and  conduction. 

In  discussing  Mr.  Gin's  paper,  F.  T.  Snyder  claims  to  have 
smelted  pure  zinc  oxide  with  an  expenditure  of  1,050  kw.-hours 
per  1,000  kg.  of  the  oxide  (2,204  lb.),  the  zinc  and  gases  leaving 
the  furnace  at  about  500°  C.,  and  Professor  Richards  shows  that 
this  is  theoretically  possible,  in  view  of  the  low  temperature  at  which 
the  products  leave  the  furnace. 

Comparing  these  very  different  figures,  it  may  be  stated  that 
the  lower  values  given  by  Mr.  Snyder,  Casaretti  and  Bertani, 
and  the  author,  are  probably  sufficient  for  the  simple  distillation 
of  zinc  ores  or  zinc-lead  ores  of  the  composition  stated,  when  the 
gases  leave  the  furnace  at  a  low  temperature,  and  when  the  resulting 
slags  are  easily  fusible.  In  the  regular  smelting  of  zinc  ores,  when 
it  is  desired  to  obtain  the  metal  in  the  molten  state,  the  gases  will 
usually  leave  the  furnace  at  a  higher  temperature,  the  slag  will 
frequently  be  somewhat  refractory  and  the  smelting  cannot  be  con- 
ducted in  so  economical  a  manner.  Under  these  conditions,  the 
expenditure  of  electrical  energy  will  agree  more  closely  with  the  cal- 
culations of  Mr.  Gin  or  the  figures  given  by  Salgues,  that  is  about 
1,500-1,600  kw.-hours  for  a  ton  of  zinc  ore.  The  figure  given  by 
F.  W.  Harbord,  2,078  kw.-hours  per  metric  ton  agrees  very  well 
with  this  when  it  is  remembered  that  Mr.  Harbord's  figure  includes 
the  smelting  of  the  ore  in  the  ore  furnace,  and  the  re-distillation  of 
the  blue  powder  in  a  separate  furnace.  The  energy  necessary  for 
the  latter  purpose  being  500  to  600  kw.-hours  per  ton  of  the  powder, 
which  was  about  equal  in  weight  to  the  original  ore.  It  is  probable 
that  marked  improvements  in  the  direction  of  economy  will  be 
effected  when  the  conditions  necessary  for  electric  zinc  smelting 
are  better  understood. 

Zinc  Smelting  at  McGill. — Experiments  on  the  electrical  smelt- 
ing of  zinc-ores  have  been  made  in  the  Metallurgical  Laboratory  at 
McGill  University,  for  the  Canadian  Government,  from  the  year 
1910  to  1913.  The  work  has  been  under  the  direction  of  Dr.  Eugene 
Haanel,  Director  of  Mines,  and  Mr.  W.  R.  Ingalls  the  well-known 
zinc  expert. 

Experiments  were  made  at  first  to  study  the  distillation  and  con- 
densation of  zinc  from  spelter  and  from  mixtures  of  zinc  oxide  and 
carbon,  in  small  appliances  heated  electrically.  The  next  step  was 


THE  ELECTRIC  SMELTING  OF  ZINC  325 

the  distillation  of  zinc  from  a  mixture  of  oxide  and  carbon,  and 
from  roasted  zinc-ore  and  carbon,  in  an  electrically-heated  retort 
of  somewhat  larger  dimensions;  finally  the  continuous  smelting  of 
roasted  zinc-ores  and  lead-zinc  ores  was  attempted  in  furnaces  of 
20  k.w.  to  30  k.w. 

While  the  author  is  not  at  liberty  to  give  particulars  of  these 
experiments,  it  may  be  stated  that  the  only  serious  difficulty  en- 
countered was  the  old  one  of  the  formation  of  too  large  a  proportion 
of  "blue-powder"  in  the  condenser,  and  that  this  has  been  partly 
overcome  in  the  form  of  furnace  finally  constructed.  The  smelting 
rate  in  this  furnace  was  quite  satisfactory  in  relation  to  its  size,  and 
this  supports  the  belief  that  the  electric  smelting  of  zinc-ores  will 
become  of  commercial  importance  as  soon  as  the  "blue-powder" 
difficulty  shall  have  been  more  completely  overcome. 

In  regard  to  these  experiments,  reference  may  be  made  to  a  paper 
by  Mr.  Ingalls  on  the  "Electric  Smelting  of  Zinc  Ores."1 

As  the  experiments  were  made  on  an  increasing  scale,  they  finally 
outgrew  the  facilities  of  a  College  Laboratory,  and  are  therefore  to 
be  continued  in  the  electric  zinc-smelting  plant  at  Nelson  in  British 
Columbia. 

PRODUCTION  OF  LIQUID  ZINC 

A  notable  defect  in  the  electric  smelting  of  zinc  ores  is  the  difficulty 
experienced  in  obtaining  the  distilled  zinc  in  the  liquid  state.  In 
the  older  processes  a  large  proportion  of  the  zinc  condenses  as  a 
liquid  in  the  clay  condensers,  which  are  fitted  to  the  end  of  each 
retort,  and  are  hot  enough  to  keep  the  metajl  liquid,  a  small  portion 
passing  on  to  the  cooler  "prolong"  and  condensing  in  this  as  a 
metallic  powder;  but  when  zinc  ores  are  smelted  electrically,  very 
little  liquid  metal  is  commonly  obtained,  nearly  all  the  zinc  being 
in  the  state  of  powder. 

In  the  condensation  of  zinc  vapor,  all  that  condenses  at  tempera- 
tures above  419°  C.  will  be  in  the  molten  condition  (whether  in 
drops  or  collected  together),  but  below  that  temperature  the  zinc 
will  form  as  a  solid  powder  analogous  to  snow  or  hoar  frost.2  The 
relative  amount  of  zinc  which  condenses  to  a  liquid  and  to  a  solid 

XW.  R.  Ingalls,  Journ.  Can.  Inst.  Min.  Eng.,  xv,  1912,  p.  101.  Eng.  and 
Min.  Journ.,  Aug.,  1912,  p.  481. 

2  Zinc  snow  or  hoar  frost  is  known  as  "blue  powder,"  and  it  is  impossible  to 
cause  this  to  run  together  by  heating  it  to  the  melting  temperature. 


326  THE  ELECTRIC  FURNACE 

depends  on  the  extent  to  which  the  zinc  vapor  is  diluted  with  other 
gases;  the  greater  the  dilution,  the  larger  will  be  the  proportion  of 
the  solid.  Thus  in  the  condensation  of  moisture  from  air,  rain  or 
dew  is  formed  when  a  moist  wind  is  cooled,  but  from  nearly  dry  air 
no  moisture  may  condense  until  the  air  is  cooled  below  the  freezing- 
point,  and  the  moisture  will  then  condense  as  snow  or  hoar  frost. 

The  vapor  pressure  of  zinc  at  its  melting-point  is  only  a  fraction 
of  a  millimeter  of  mercury  (an  atmosphere  being  760  mm.),  and 
consequently,  even  if  the  zinc  vapor  were  diluted  with  two  or  three 
times  its  volume  of  inert  gases,  the  amount  of  zinc  that  would 
condense  as  a  solid  would  be  far  less  than  i  per  cent,  of  the  whole, 
and  would  be  too  little  to  be  of  any  importance.  This  calculation 
is  made  on  the  assumption  that  the  condensation  of  zinc  vapor  at 
any  temperature  continues  until  the  residual  vapor  corresponds  in 
amount  to  the  vapor  pressure  of  zinc  at  that  temperature,  and  in 
practice,  as  the  gases  move  quickly  through  a  condenser,  it  may  be 
that  the  condensation  is  not  complete  at  each  temperature,  and  that 
the  rapidly  cooling  gases  contain,  at  the  melting-point  of  zinc,  more 
zinc  vapor  than  would  correspond  to  the  actual  vapor  pressure  of 
zinc  at  that  temperature,  and  in  this  way  a  somewhat  larger  propor- 
tion of  the  zinc  would  condense  as  a  solid. 

The  condensation  of  zinc  vapor  as  a  solid  is,  however,  only  a 
very  small  part  of  the  trouble.  The  real  difficulty  lies  in  the  manner 
of  condensation  of  the  zinc  at  temperatures  above  its  melting-point. 
The  zinc  vapor  normally  condenses  on  the  walls  of  the  condenser 
forming  drops  of  zinc  which  grow  until  they  become  large  enough  to 
run  down  into  the  molten  zinc  lying  in  the  bottom  of  the  condenser. 
This  method  of  condensation  can  be  observed  in  the  case  of  water 
in  a  surface-cooled  condenser  made  of  glass. 

An  entirely  different  result  is  obtained  when  the  zinc  vapor  con- 
denses, not  on  the  walls  of  the  condenser,  but  as  a  cloud  of  minute 
drops  floating  in  the  gases  that  fill  the  condenser.  These  drops 
are  ultimately  deposited  from  the  gases,  and  accumulate  at  the 
bottom  and  on  the  walls  of  the  condenser,  but  unfortunately  they 
do  not  coalesce  with  each  other  even  when  they  are  at  a  red  heat 
and  protected  from  oxidizing  gases.  This  form  of  condensation 
is  familiar  to  everyone,  as  it  is  seen  in  the  formation  of  the  minute 
drops  of  water  forming  the  "steam"  which  comes  from  the  spout 
of  a  kettle,  and  the  "fog"  produced  when  moist  air  is  cooled  below 
its  point  of  saturation.  In  the  zinc-condenser  it  is  probable  that 
both  methods  of  condensation  are  always  in  operation,  but  the  mist 


THE  ELECTRIC  SMELTING  OF  ZINC  327 

of  floating  droplets  not  only  lessens  the  amount  of  vapor  that  is 
available  for  condensing  on  the  walls,  but  interferes  with  the  normal 
condensation  by  covering  up  the  growing  droplets. 

The  minute  particles  of  zinc  that  form  within  the  mass  of  the  gas 
fall  to  the  bottom  of  the  condenser  before  they  attain  any  considera- 
ble size,  and  being  covered  up  with  many  other  particles,  they  never 
become  large  enough  to  coalesce,  and  are  finally  raked  out  as  a 
mass  of  minute  spheres  of  zinc.  This  product  which  should  really 
be  called  "grain-zinc,"  is  always  included  with  the  zinc  snow 
(deposited  below  the  melting-point),  in  the  general  term  "blue 
powder." 

The  conditions  that  lead  to  one  or  the  other  method  of  condensa- 
tion may  be  stated  as  follows: 

(1)  To  obtain  surface  condensation  it  is  necessary  for  the  con- 
denser to  have  a  sufficiently  large  surface  in  relation  to  its  volume 
and  to  the  amount  of  zinc  to  be  condensed,  and  that  the  condensing 
surface  shall  be  cool  enough  for  rapid  condensation,  but  still  above 
the  melting-point  of  zinc.     If  the  condenser  is  a  large  chamber,  this 
condition  will  not  be  fulfilled  because  the  gases  will  in  general  be 
remote  from  the  walls,  and  the  zinc  will  condense  as  a  mist  of  float- 
ing droplets. 

(2)  The  presence  in  the  furnace-gases  of  floating  particles  such  as 
zinc  oxide,  carbon,  silica  or  ore-dust  will  interfere  with  the  normal 
condensation,  both  by  furnishing  points  on  which  the  zinc-mist 
can  form,  just  as  the  particles  of  water  in  a  London  fog  form  on 
floating  specks  of  dust  and  soot,  and  by  covering  the  surface  of  the 
zinc  droplets  on  the  condenser  walls  and  so  preventing  their  growth. 

(3)  The  metal  zinc  is  very  easily  oxidized  at  temperatures  above  a 
red  heat,  not  only  by  free  oxygen,  but  even  by  carbon  dioxide  and 
water  vapor.     The  presence  of  carbon  dioxide  in  excess  of  about 
i  per  cent,  interferes  very  seriously  with  the  normal  condensation  of 
zinc,  by  forming  floating  particles  of  zinc  oxide  which  act  as  nuclei 
for  the  formation  of  zinc-mist,  and  by  covering  with  oxide  the  drop- 
lets growing  on  the  condenser  walls,  thus  interfering  with  their 
growth,  and  preventing  their  coalescence. 

(4)  Diluting  gases,  even  when  neutral,  interfere  with  the  normal 
condensation  of  the  zinc:  (a)  By  lowering  the  temperature  at  which 
the  zinc  can  be  condensed;  (b)  by  lessening  (slightly)  the  amount  of 
zinc  that  can  be  condensed  above  the  melting  point;  (c)  by  interfer- 
ing mechanically  with  the  transfer  of  the  zinc  vapor  to  the  condenser 
walls.    If  there  were  no  diluting  gases,  the  zinc  vapor  would  come  into 


328  THE  ELECTRIC  FURNACE 

direct  contact  with  the  walls,  and  would  condense  there  rapidly  at  any 
temperature  below  its  boiling-point  (930°  C.),  fresh  vapor  moving  up 
to  the  walls  as  fast  as  the  first  layer  is  condensed,  but  any  diluting 
gases  will  remain  as  a  barrier  between  the  walls  and  the  uncondensed 
vapor,  and  further  condensation  can  only  take  place  as  the  result  of 
diffusion  or  convection  of  the  vapor  to  the  walls.  The  speed  of  this 
diffusion  and  convection  is  a  function  of  the  temperature,  and  thus 
the  dilution  of  the  vapors,  by  lowering  the  temperature  of  condensa- 
tion, decreases  the  rate  at  which  zinc  can  travel  to  the  walls  and  so 
interferes  still  further  with  its  normal  condensation. 

The  conditions  for  the  condensation  of  zinc  in  the  molten  state 
may  be  summarized  as  follows:  The  gases  entering  the  condenser 
must  be  as  free  as  possible  from  oxidizing  gases  and  floating  particles, 
and  they  should  contain  a  large  proportion  of  zinc.  The  condenser, 
also,  must  be  suitably  designed  to  afford  convenient  condensing 
surfaces.  The  bearing  of  this  on  the  electric  smelting  of  zinc  ores 
may  now  be  considered. 

When  roasted  zinc  ores  are  heated  with  coal  in  a  retort  or  closed 
electric  furnace,  various  gases  such  as  steam,  hydrocarbons,  carbon 
dioxide  and  carbon  monoxide  are  given  off  as  the  charge  is  heated 
to  900°  or  i, 000°  C.  Iron  oxide,  if  present,  will  be  reduced  and  even 
some  of  the  zinc  oxide  may  be  reduced  to  metal  below  its  boiling 
temperature — 930°  C. — according  to  the  equation, 

ZnO+CO  =  Zn+CO2. 

Above  1,000°  C.,  the  only  reaction  that  can  take  place  is  expressed 
by  the  equation,  ZnO+C  =  CO+Zn,  and  we  may  therefore  consider 
that  zinc  and  carbon  monoxide  are  given  off  in  this  proportion — that 
is,  equal  volumes,  if  we  assume  that  zinc  is  a  perfect  monatomic  gas 
at  this  temperature.  The  proportion  of  zinc  may  be  even  higher 
than  this  during  the  period  of  active  distillation,  on  account  of 
zinc  which  had  already  been  reduced  and  condensed  (temporarily) 
in  the  cooler  parts  of  the  charge. 

The  distillation  of  the  zinc  takes  place  regularly,  owing  to  the 
gradual  passage  of  heat  through  the  retort,  and  the  gases  are  free 
from  dust.  Under  these  conditions  a  satisfactory  condensation  is 
obtained,  some  90  per  cent,  of  the  zinc  being  in  the  molten  condition'. 
This  operation  can  be  carried  out  equally  well  in  an  electrically 
heated  furnace  (though  it  is  lessveasy  to  arrange  for  the  equable  heat- 
ing of  the  charge)  and  a  good  condensation  of  the  zinc  can  be  ob- 


THE  ELECTRIC  SMELTING  OF  ZINC  329 

tained.  The  process  is  intermittent,  however,  like  the  distillation  of 
zinc-ore  in  retorts,  a  charge  being  placed  in  the  furnace,  gradually 
heated  and  the  zinc  distilled;  after  which  the  spent  charge  is  with- 
drawn, and  a  fresh  charge  introduced. 

One  of  the  main  features  of  electrical  smelting  is  the  possibility 
of  avoiding  this  step-by-step  process,  and  making  the  smelting  proc- 
ess continuous.  In  order  to  obtain  the  full  advantages  of  electrical 
smelting,  it  is  essential  that  the  furnace  shall  operate  continuously 
with  continuous  or  at  least  frequent  introductions  of  charge,  and  with 
continuous  distillation  and  condensation  of  the  zinc.  If  now  we  sup- 
pose that  the  ore  and  coal  are  charged  continuously  into  the  furnace, 
the  escaping  gases  will  contain,  in  addition  to  the  equal  volumes 
of  zinc  vapor  and  carbon  monoxide,  a  considerable  amount  of 
additional  carbon  monoxide  and  other  gases  which  are  liberated 
at  lower  temperatures.  This  additional  gas  dilutes  the  zinc  vapor 
and  oxidizes  part  of  it,  giving  rise  to  large  quantities  of  blue 
powder  in  the  condenser. 

Some  experimenters  have  considered  that  the  oxidation  of  the  zinc 
by  CO2  was  the  essential  difficulty,  and  have  provided  carbon  filters, 
heated  to  1,000°  C.,  through  which  the  gases  could  pass  on  their  way 
to  the  condenser.  In  spite  of  published  statements,  the  writer 
doubts  whether  this  method  has  produced  any  considerable  improve- 
ment in  the  condensation.  Another  method  consists  in  heating 
the  ore-charge  in  a  closed  vessel  nearly  to  the  reduction  temperature 
before  charging  it  into  the  furnace,  thus  removing  a  large  part  of  the 
gases  that  are  evolved  at  a  low  temperature  and  correspondingly 
increasing  the  concentration  of  the  zinc. 

It  appears  better  to  remove  the  gases  as  far  as  possible  in  this  way 
than  to  leave  them  in  and  merely  to  deoxidize  them  with  a  carbon 
filter.  In  this  connection  it  must  not  be  forgotten  that  in  the  zinc- 
retort  the  ore  is  mixed  with  about  50  per  cent,  of  its  weight  of  coal, 
while  in  the  electric  furnace  only  about  15  per  cent,  to  20  per  cent, 
can  be  used,  as  any  more  would  remain  as  an  infusible  residue, 
and  would  choke  the  furnace.  The  excess  of  carbon  in  the  zinc- 
retort  doubtless  assists  in  keeping  the  gases  thoroughly  deoxidized, 
and  in  the  electric  furnace  it  is  probably  desirable  on  this  account  to 
have  some  heated  carbon  in  the  furnace  in  contact  with  the  gases. 

If  the  above  conditions  were  fulfilled,  there  would  still  remain 
some  reasons  why  the  condensation  in  electric  smelting  may  be 
less  perfect  than  in  the  regular  practice: 

(a)  In  the  retort  there  is  probably  an  accumulation  of  metallic 


330  THE  ELECTRIC  FURNACE 

zinc  in  the  charge  at  the  time  when  distillation  commences.  This 
will  not  usually  take  place  in  continuous  electric  smelting. 

(b)  It  is  very  difficult  in  electric  smelting  to  get  the  same  regu- 
larity of  operation  as  in  the  zinc  retort. 

(c)  As  the  scale  of  operations  is  larger  in  electric  smelting,  it 
becomes  more  difficult  to  design  a  condenser  having  the  same  ratio  of 
surface  to  volume  as  in  regular  practice. 

(d)  In  electric  smelting  it  is  difficult  to  avoid  the  production  of 
zones  of  very  high  temperature  in  which  silica  and  other  oxides  may 
be  reduced,  yielding  volatile  products  which  may  interfere  with  the 
condensation  of  the  zinc. 

The  reduction  of  zinc  oxide  to  metal  is  represented  by  the  equation, 

ZnO+C  =  CO+Zn, 

but  as  the  solid  carbon  cannot  come  into  sufficiently  close  contact 
with  the  solid  zinc  oxide,  it  seems  probable  that  carbon  monoxide 
is  the  actual  reducing  reagent,  the  resulting  carbon  dioxide  being 
immediately  reduced  to  monoxide  by  the  carbon. 

ZnO+CO  =  Zn+CO2 


These  reactions  can  only  take  place  at  a  high  temperature  (over 
1,000°  or  1,100°  C.),  because  at  lower  temperatures  the  normal 
proportion  of  CC>2  would  be  sufficient  to  reoxidize  the  zinc.  The 
temperature  at  which  the  reduction  of  zinc  oxide  by  carbon  begins 
has  been  determined  by  W.  McA.  Johnson,1  who  finds  it  to  vary 
between  about  1,020°  C.  and  1,080°  C.,  according  to  the  kind  of 
ore  and  the  kind  of  carbon  employed. 

The  vapor  pressure  of  molten  zinc  has  been  determined  experi- 
mentally by  C.  Bams,2  at  temperatures  up  to  the  boiling-point, 
and  the  author  has  calculated  from  these  data  the  temperatures  at 
which  the  condensation  of  zinc  will  start  from  mixtures  of  zinc  and 
carbon  monoxide  of  varying  concentration,  and  also  the  tempera- 
tures at  which  50  per  cent,  and  90  per  cent,  of  the  zinc  will  have 
condensed.  These  results  are  given  in  Table  XXI,  which  also  shows 
approximately  what  percentage  of  carbon  dioxide  would  tend  to 
form  in  the  gas  mixture  at  each  temperature,  in  consequence  of  the 
reaction  2CO  =  C02+C.  Carbon,  dioxide  could  not  actually  form 

1  W.  McA.  Johnson,  "The  Reduction  Temperature  of  Zinc  Oxide,"  Trans.  Am. 
Electrochem.  Soc.,  v,  1904,  p.  211. 

2  C.  Barus,  U.  S.  Geol.  Survey,  Bull.  102,  1893. 


THE  ELECTRIC  SMELTING  OF  ZINC 


331 


in  any  quantity,  as  it  would  immediately  oxidize  the  zinc  by  the 
reaction  CO2+Zn  =  ZnO+CO,  but  the  amount  of  carbon  dioxide 
shown  in  the  table  may  be  taken  as  a  rough  measure  of  the  tendency 
of  the  zinc  vapor  to  reoxidize.  The  two  equations  written  above 
are  equivalent  to: 

CO+Zn  =  ZnO+C, 

thus  showing  that  the  reaction  whereby  zinc  oxide  is  reduced  by 
carbon,  is  reversed  at  the  lower  temperature  necessary  for  condensa- 
tion, and  that  this  reversed  reaction  takes  place  the  more  rapidly 
as  the  gas  mixture  is  richer  in  carbon  gases. 

The  finely  divided  carbon  and  zinc  oxide  resulting  from  this 
reaction  will  tend  to  coat  the  growing  globules  of  condensed  zinc 
and  prevent  their  coalescence.  We  see  therefore  that  no  matter 
how  carefully  we  remove  CO2  and  dirt  from  the  gases  leaving  a 
smelting  furnace,  the  condensation  of  zinc  can  never  be  perfect, 
and  that  these  difficulties  will  increase  rapidly  with  dilution  of  the 
gas-mixture.  It  also  appears  desirable  for  the  condensation  to  be 
effected  as  rapidly  as  possible,  so  that  very  little  time  shall,  be 
available  for  this  reaction. 

TABLE  XXI.— CONDENSATION  OP  ZINC  FROM  MIXTURES  OP  ZINC  VAPOR 
AND  CARBON   MONOXIDE 


Condensation 

50  per  cent. 

90  per  cent. 

Zinc  in 

starts 

condensed 

condensed 

mixture 
Per  cent. 

Temp. 

C02 
Per  cent. 

Temp. 

C02 
Per  cent. 

Temp. 

C02 

Per  cent. 

IOO 

930°  C. 

o.o 

930°  C. 

o.o 

930°  C. 

o.o 

90 

918° 

0-3 

910° 

o-S 

860° 

2.8 

80 

908° 

0.6 

890° 

i-3 

v        821° 

5-6 

70 

895° 

1.  1 

872° 

2-3 

788° 

IO.  I 

60 

880° 

1.8 

853° 

3-3 

758° 

18.7 

50 

865° 

2-S 

832° 

4-7 

737° 

25 

40 

846° 

3-7 

810° 

6.8 

718° 

33 

30 

823° 

5-2 

783° 

ii.  5 

690° 

45 

20 

792° 

9-6 

750° 

21 

660° 

55 

IO 

743° 

23-7 

705° 

37 

620° 

70 

Blue  Powder. — This  consists,  as  has  been  stated,  of  zinc-snow  or 
hoar-frost  and  also  of  grain  zinc  which  is  analogous  to  hail.  This 
material  does  not  run  together  when  heated  to  its  melting-point,  even 
when  heated  in  neutral  or  reducing  gases,  and  it  is  generally  supposed 
that  a  film  of  oxide  or  other  substance  is  the  cause  of  the  non- 
coalescence  of  the  zinc  particles.  Blue  powder  is  a  constant  by- 


332  THE  ELECTRIC  FURNACE 

product  of  zinc  distillation,  and  is  usually  worked  over  with  additions 
of  zinc  ore,  although  some  can  be  sold  for  use  as  a  reducing  agent  in 
indigo  dyeing,  !»and  gold  cyaniding.  At  one  time  blue  powder  was 
heated  and  pressed,2  yielding  molten  zinc,  but  the  practice  has  been 
discontinued. 

The  powder  can  be  melted  with  the  aid  of  a  suitable  flux  such  as 
zinc  chloride,  but  a  considerable  proportion  of  the  flux  is  required. 
Another  process  consists  in  adding  the  powder  to  an  electrolytic 
bath  of  fused  zinc  chloride,  having  a  carbon  anode.  Any  oxide  of 
zinc  will  dissolve  in  the  chloride  and  will  be  electrolyzed,  while  the 
metallic  zinc,  freed  from  oxide  or  other  coating,  will  run  together. 

The  electrical  energy  needed  amounts  to  about  i/io  kw.-hour  per 
pound  of  zinc,  and  the  loss  of  chloride  is  about  1/25  lb.3  The  proc- 
ess is  apparently  cheaper  than  that  of  redistilling  the  powder,  and 
may  overcome  the  objection  to  the  formation  of  blue  powder  in 
electric  smelting,  but  electric-furnace  blue  powder  often  contains  a 
proportion  of  fine  ore  dust  and  carbon,  which  will  tend  to  choke  the 
bath  and  increase  the  consumption  of  chloride. 

The  following  electric  zinc  processes  and  furnaces  may  be  described 
as  illustrating  the  directions  in  which  improvements  have  been  at- 
tempted during  recent  years: 

The  Cote-Pierron  zinc  process  and  furnaces. 

The  Imbert  zinc  process  and  Thomson-FitzGerald  furnace. 

The  Johnson  zinc  furnace. 

The  Thierry  zinc  furnace. 

The  Louvrier-Louis  zinc  furnace. 

The  Cote-Pierron  process4  is  intended  for  the  treatment  of  mixed 
sulphides  of  lead  and  zinc.  The  ore  is  not  roasted  to  oxide  but  treated 
in  the  raw  state  with  enough  iron  to  combine  with  the  sulphur. 
The  lead  is  reduced  first  at  a  moderate  temperature,  and  then  the 
temperature  is  raised  for  the  reduction  of  the  zinc,  which  takes  place 
according  to  the  equation 

ZnS+Fe  =  FeS+Zn. 

1  W.  R.  Ingalls,  Metallurgy  of  Zinc,  First  Ed.,  p.  667. 

2  W.  R.  Ingalls,  Metallurgy  of  Zinc,  First  Ed.,  p.  527. 

3  W.  F.  Blecker,  Electrolytic  Reduction  of  Blue  Powder,  Trans.  Am.  Electro- 
chem.  Soc.,  xxi,  1912,  p.  359. 

4  E.  Fleurville,  La  Houille  Blanche,  vol.  vii  (1908),  p.  273;  Electrochem.  and 
Met.  Ind.,  vol.  vii  (1909),  p.  468. 

Dr.  Eugene  Haanel,  "Recent  Advances  in  the  Construction  of  Electric  Fur- 
naces for  the  Production  of  Pig-iron,  Steel  and  Zinc."  Ottawa,  1910. 


THE  ELECTRIC  SMELTING  OF  ZINC 


333 


The  furnace,  shown  in  Fig.  133,  is   cylindrical  and  is  lined  with 
graphite,  but  the  roof  and  condenser  are  built  with  bricks.     A  coni- 


O  s\NN\  >xy  S.NV  N  y "  y^^  

////////^^^ 


FIG.  133. — C6te-Pierron  furnace. 

cal  electrode,  b,  forms  part  of  the  bottom  of  the  furnace,  which  is 
connected  to  one  pole  of  the  electrical  supply,  and  a  movable  electrode 
enters  through  the  roof  of  the  furnace.  The  heating  is  effected  by  re- 


334  THE  ELECTRIC  FURNACE 

sistance  when  a  low  temperature  is  needed  for  the  reduction  of  the 
lead,  and  by  an  arc,  when  a  higher  temperature  is  needed  for  the  re- 
duction of  the  zinc;  the  electrode  being  lifted  out  of  the  slag.  The 
charge  of  ore  and  iron  is  preheated  on  the  roof  of  the  furnace,  and 
introduced  by  the  openings,  e\  the  reduced  lead  and  the  slag  being 
tapped  out  by  the  hole,  /.  The  volatilized  zinc  escapes  by  the  pas- 
sage, g,  and  condenses  partly  in  h,  and  partly  in  the  condenser  /, 
which  is  kept  full  of  pieces  of  red  hot  carbon  to  provide  an  extensive 
surface  for  its  condensation.  The  zinc  collects  at  /,  and  the  column  of 
carbon  can  be  heated  by  admitting  air  at  j. 

In  treating  an  ore  containing  lead  and  zinc,  the  lead  is  reduced 
first,  at  a  low  temperature,  and  tapped  out  of  the  furnace  before  the 
temperature  is  raised  for  the  reduction  of  the  zinc.  In  this  way  there 
is  less  contamination  of  the  zinc  by  volatilized  lead. 

As  zinc  is  the  only  gaseous  product  of  the  reaction  between  zinc 
sulphide  and  iron,  it  is  undiluted  by  carbon  monoxide  or  other  gases, 
and  should  condense  readily  to  the  molten  condition.  The  process 
avoids  the  necessity  of  roasting  the  ore,  but  incurs  a  larger  expense 
for  metallic  iron  to  carry  out  the  reduction;  1,800  Ib.  of  iron  being 
needed  to  produce  2,000  Ib.  of  zinc,  and  in  addition,  about  600  Ib. 
for  every  2,000  Ib.  of  lead. 

The  C6te-Pierron  process  has  been  tried  on  a  large  scale  in  the 
south  of  France,  but  does  not  appear  to  have  made  much  headway. 

The  Imbert  Zinc  Process.1 — This  resembles  the  Cote-Pierron  proc- 
ess as  it  depends  on  the  reaction  between  zinc  sulphide  and  metallic 
iron. 

It  consists  in  fusing  the  zinc  ore  with  suitable  fluxes,  and  then 
pouring  into  the  fused  mass  the  necessary  amount  of  melted  pig-iron. 
The  reaction  takes  place  immediately  yielding  zinc  and  an  iron  matte. 
A  mixture  of  one  part  of 'ferric  oxide  and  three  parts  of  iron  sulphide 
form  a  fluid  bath  at  a  temperature  between  1,000°  C.  and  1,100°  C., 
and  will  "dissolve"  six  parts  of  blende.2  When  this  mixture  is 
treated  with  iron,  the  zinc  is  liberated,  nearly  completely,  as  vapor, 
and  the  residue  consists  of  a  slag  and  a  matte  of  iron  sulphide  which 
can  be  used  for  the  next  operation.  This  process  is  not  necessarily 

1  A.  H.  Imbert,  U.  S.  patent  85,579,  Dec.,  1907.     M.  Imbert's  original  idea  was 
to  use  metallic  copper  as  the  reagent;  the  copper  being  recovered  from  the  result- 
ing sulphide  by  the  usual  methods  and  used  over  again.     A.  H.  Imbert,  U.  S. 
patent  807,271,  Dec.,  1905. 

2  F.  A.  J.  FitzGerald,  "A  New  Electric  Resistance  Furnace,"  Trans.  Am. 
Electrochem.  Soc.,  xix,  1911,  p.  273. 


THE  ELECTRIC  SMELTING  OF  ZINC  335 

electrothermic,  and  was  tried  at  first  in  gas-fired  or  oil-fired  furnaces, 
but  as  the  reaction  must  be  effected  in  closed  vessels,  from  which  air 
must  be  excluded,  electrical  heating  is  particularly  adaptable. 

The  Imbert  process  has  been  tried  in  upper  Silesia  in  the  Thomson- 
Fitz Gerald  resistor  furnace,1  which  is  shown  diagrammatically  in  Fig. 
134.  A  permanent  carbon  resistor,  RR,  forms  the  roof  of  the  fur- 
nace, thus  heating  the  charge  by  radiation.  The  resistor  is  composed 
of  a  number  of  wedge-shaped  carbon  rods  having  their  narrow  edges 
alternately  up  and  down.  The  bars  having  their  edges  up  are  sup- 
ported by  the  sides  of  the  furnace,  while  the  other  bars  are  dropped  in 
between,  making  electrical  contact  with  them.  This  arrangement 
provides  a  constant  degree  of  contact  between  the  bars,  in  spite  of 
any  expansion  or  contraction,  the  upper  rods  rising  or  falling  to  take 
up  such  changes.  The  heat  is  produced  mainly  at  the  contacts  be- 


FIG.  134. — Resistor  zinc  furnace. 

tween  the  wedges,  but  the  whole  series  of  bars  will  become  strongly 
heated,  and  will  radiate  the  heat  into  the  cavity  of  the  furnace. 

It  is  of  course  essential  to  maintain  a  non-oxidizing  atmosphere  in 
the  furnace,  or  the  carbon  rods  would  soon  burn  up.  The  resistor 
bars  are  covered  with  a  layer  of  burnt  magnesite,  M,  which  serves  to 
retain  the  heat  and  exclude  the  air. 

In  operating  the  Imbert  process,  the  resistor  furnace  formed 
the  reaction  chamber  into  which  were  charged  the  molten  iron  and 
the  "dissolved"  zinc  ore.  The  zinc  vapor  passed  out  to  a  condenser 
and  the  slag  and  matte  were  then  tapped  from  the  furnace. 

The  first  furnace  built  on  this  plan  had  a  capacity  of  550  Ib.  of 
pig-iron  and  employed  40  kw.  The  radiation  loss  at  1,300°  C.  was 
1 8  kw.,  showing  an  efficiency  of  55  per  cent.  A  larger  furnace  of 

1  F.  A.  J.  FitzGerald,  "A  New  Resistor  Furnace,"  Met.  and  Chem.  Eng., 
viii,  1910,  p.  317. 

Radiation  Resistor  Furnace,  Met.  and  Chem.  Eng.,  viii,  1910,  p,  289. 


336 


THE  ELECTRIC  FURNACE 


1 50  kw.  was  then  built,  having  the  carbon  resistor  arched  from  end  to 
end  of  the  furnace,  as  in  Fig.  15.  This  radiated  33  kw.  at  1,250°  C., 
giving  an  efficiency  of  78  per  cent,  when  worked  at  150  kw. 

The  Johnson  zinc  furnace1  is  shown  in  Fig.  135. 2  It  consists  of  a 
smelting  chamber,  C,  provided  with  three  carbon  electrodes,  a  charg- 
ing chute  and  two  tap  holes.  Two  of  the  electrodes  are  movable, 
entering  through  the  roof,  and  the  third  forms  part  of  the  bottom  of 
the  furnace.  The  furnace  contains  molten  lead,  L,  which  has  been 
reduced  from  the  ore,  and  above  that  a  layer  of  molten  slag,  S,  on 
which  the  ore  charge  rests.  Heat  is  produced  partly  in  the  slag  and 
partly  in  the  charge  around  the  upper  electrodes.  Roasted  ore  is 
used,  consisting  mainly  of  oxide  of  zinc,  and  of  lead  when  present, 
together  with  oxide  of  iron  and  gangue  matter.  The  roasted  ore, 


FIG.  135. — Johnson  zinc  furnace. 


mixed  with  carbon  for  its  reduction  and  lime  or  other  flux,  is  charged 
by  means  of  the  hopper  and  chute.  The  liberated  zinc  vapor,  with 
diluting  gases,  pass  off  by  a  channel  and  descend  through  a  tower,  T, 
filled  with  broken  carbon  which  is  heated  to  at  least  1,000°  C.  by 
an  electric  current  which  passes  between  the  electrodes,  E  E. 
This  serves  to  reduce  any  carbon  dioxide  to  carbon  monoxide, 
and  so  to  avoid  oxidation  of  the  zinc  vapor.  The  gases  then 
enter  the  condenser,  F  F,  which  is  kept  above  the  melting-point 
of  zinc.  It  is  provided  with  baffle  walls,  which  are  air-cooled  (by 
enclosed  pipes)  to  remove  the  heat  of  condensation  of  the  zinc,  which 
collects  in  the  molten  condition  at  Z.  No  preheater  is  provided  for 

1  J.  W.  Richards,  Trans.  Am.  Electrochem.  Soc.,  xix,  1911,  p.  311. 

2  W.  McA.  Johnson,  U.  S.  patent  964,268,  July,  1910. 


THE  ELECTRIC  SMELTING  OF  ZINC  337 

the  removal  of  gases  from  the  ore  charge,  and  the  reducing  filter  does 
not  overcome  the  dilution  of  the  zinc  vapors.  The  carbon  filter 
appears,  moreover,  in  view  of  the  experiments  at  McGill  University, 
to  interfere  with  the  subsequent  condensation  of  the  zinc  vapor. 
In  an  electrically  heated  filter  there  will  probably  be  points  suffi- 
ciently hot  to  produce  volatilization  of  carbon,  silica  or  other 
substances  which  will  give  trouble  in  the  condenser. 

In  more  recent  accounts,1  no  details  of  the  design  are  given,  but 
it  is  stated  that  the  ore  charge  is  heated  to  about  900°  C.  in  a  con- 
tinuous preheater,  and  the  photographs  suggest  that  the  zinc  is 
condensed  in  a  number  of  iron  pipes.  A  considerable  improvement 
has  been  effected  in  regard  to  the  condensation,  but  it  is  necessary 
to  smelt  slowly  to  secure  a  good  condensation,  and  this  increases  the 
cost  of  the  process.  The  amount  of  electrical  energy  needed  is 
expected  to  be  800  kw.-hours  per  short  ton  of  a  30  per  cent,  roasted 
ore  and  1,400  kw.-hours  for  a  70  per  cent,  ore  when  working  on  the 
large  scale,  but  the  consumption  in  the  36-kw.  experimental  furnace 
in  one  test  was  1,490  kw.-hours  for  a  short  ton  of  a  39  per  cent, 
roasted  ore.  A  typical  slag  analysis  is:  40  per  cent.  SiO2,  22  per 
cent.  CaO,  2  per  cent.  MgO  10  per  cent.  FeO,  i  per  cent.  MnO,  10 
per  cent.  A^Os,  2  per  cent.  ZnO,  0.15  per  cent.  Cu,  0.05  per  cent. 
Pb  and  0.30  oz.  Ag  per  ton.  The  ore  is  roasted  until  the  sulphur  is 
reduced  to  from  3  per  cent,  to  6  per  cent.,  depending  on  the  amount 
of  copper  present,  and  the  silver  is  collected  in  the  lead  bullion,  or 
if  no  lead  is  present,  in  a  matte.  On  account  of  the  strong  reduction 
of  the  electric  furnace,  this  matte  is  lower  in  lead  tha'n  is  usual  in  a 
lead  blast-furnace;  a  typical  matte  containing  45  per  cent,  of  iron, 
25  per  cent,  of  copper  and  29  per  cent,  of  sulphur. 

A  paper  by  W.  McA.  Johnson,2  which  has  just  appeared,  gives 
further  particulars  of  the  success  recently  obtained  by  the  Continu- 
ous Zinc  Smelting  Co.  with  the  Johnson  zinc  smelting  process.  The 
zinc  condensed  amounts  to  more  than  80  per  cent,  of  the  zinc  in  the 
ore,  and  70  or  80  per  cent,  of  this  is  in  the  form  of  spelter.  The 
short  ton  of  preheated  ore  takes  from  938  to  1748  k.w.  hours  in 
different  runs,  depending  partly  on  the  zinc  contents. 

Mr.  Johnson  considers  the  process  especially  suitable  for  low- 
grade  zinc  ores  containing  considerable  quantities  of  lead,  copper 
gold  and  silver,  which  will  be  recovered  as  by-products. 

1  Met.  and  Chem.  Eng.,  x,  1912,  pp.  281  and  537. 

2  W.  McA.  Johnson,  "The  Art  of  Electric  Zinc  Smelting,  Am.  Electrochem. 
Soc.  xxiv,  1913. 

22 


338 


THE  ELECTRIC  FURNACE 


The  Thierry  zinc  furnace,1  shown  in  Fig.  136,  is  intended  for  the 
treatment  of  pure  zinc  oxide,  free  from  gangue  or  slag-forming  mate- 
rial; the  oxide  having  been  obtained  by  the  Wetherill  or  other  proc- 
ess. The  furnace  is  provided  with  a  carbon  resistor  (shaded  darkly) 
on  which  rests  the  mixture  of  zinc  oxide  and  carbon,  O.  Additional 
quantities  of  ore  mixture  are  being  heated  in  trays,  TT,  before  being 
charged  through  the  central  hopper,  H.  Heat  for  the  reduction  of 
the  zinc  is  produced  by  the  passage  of  an  electric  current  through  the 
resistor.  The  products  of  the  reaction  (zinc  vapor  and  carbon  mon- 
oxide), pass  into  the  lateral  condensers,  CC,  which  contain  coarse 
coke.  The  carbon  of  the  resistor  and  adjacent  coke  at  a  high  tem- 
perature serves  to  reduce  any  volatilized  zinc  oxide  or  carbon  dioxide, 


FIG.  136. — Thierry  zinc  furnace. 

and  the  cooler  coke  provides  a  large  surface  on  which  the  zinc  can 
condense.  The  uncondensed  gases  pass  through  the  cooler  "pro- 
longs," PP,  where  most  of  the  remaining  zinc  is  condensed  before 
the  carbon  monoxide  escapes  and  is  burnt.  Tapping  holes  are  pro- 
vided for  withdrawing  the  molten  zinc,  and  fires,  if  necessary,  for 
keeping  the  condenser  above  the  melting-point  of  zinc.  The  inventor 
claims  to  have  run  such  a  furnace  continuously  for  weeks,  producing 
zinc  at  the  rate  of  about  7  5  Ib.  per  hour.  It  seems  very  probable  that 
the  condensers  would  become  choked  with  zinc-powder,  and  would 
need  frequent  poking  to  keep  a  clear  passage  through  the  coke. 

Louvrier-Louis  Zinc  Furnace.— This  is  shaped  like  an  iron  blast- 
furnace and  is  provided  with  a  number  of  carbon  electrodes  built  into 

1  C.  V.  Thierry,  of  Paris,  U.  S.  patent  1,030,349-50,  June,  1912,  Met.  and 
Chem.  Eng.,  x,  1912,  p.  490. 


THE  ELECTRIC  SMELTING  OF  ZINC  339 

the  walls  of  the  boshes,  and  one  in  the  bottom  of  the  crucible;  two- 
phase  or  three-phase  current  being  employed.  In  this  way  the  charge 
is  heated  moderately,  for  the  distillation  of  the  zinc;  and  then  more 
strongly  to  fuse  the  residues.  The  zinc  vapors  escape  from  the 
charge  and  pass  to  the  condenser  through  lateral  openings  in  the  lower 
part  of  the  stack;  a  ledge  being  placed  above  the  openings  so  that  the 
ore  shall  not  enter  them.  The  upper  part  of  the  shaft  serves  to  pre- 
heat the  charge,  and  to  remove  part  of  the  carbon  monoxide,  which 
passes  upward  depositing  the  accompanying  zinc  on  the  descending 
ore,  as  in  Snyder's  furnace,  Fig.  132. 

ELECTRIC  SMELTING  OF  OTHER  METALS 

In  regard  to  the  application  of  the  electric  furnace  to  the  produc- 
tion of  the  common  metals  from  their  ores  the  following  general 
statements  may  be  made: 

(1)  The  production  of  iron  and  steel  has   already  been  fully 
discussed. 

(2)  The  electric  smelting  of  zinc  ores  has  always  appeared  par- 
ticularly hopeful  in  view  of  the  peculiar  conditions  necessary  for 
the  reduction  of  this  metal. 

(3)  Ores  containing  copper,  lead,  tin,  nickel  or  other  metals  in 
an  oxidized  state  are,  at  present,  smelted  with  carbonaceous  fuel 
which  serves  to  reduce  the  oxides  to  metals,  and  to  furnish  heat  for 
fusing  the  metal  and  slag,  just  as  in  smelting  iron-ores.     These 
operations  can  be  carried  out  equally  well,  and  often  somewhat 
better,  in  an  electric  furnace,  and  this  method  can  be  adopted  when- 
ever the  cost  of  electrical  power  is  sufficiently  low  and  that  of  fuel 
sufficiently  high    to  render  the    change   commercially   desirable. 
In  smelting  such  ores,  it  is  essential  to  reduce  the  oxides  of  copper, 
lead,  tin  or  nickel  to  the  metallic  state,  but  not  to  reduce  the  oxide 
of  iron  which  usually  accompanies  them;  the  iron  not  being  desired 
in  the  reduced  metal,  and  the  iron  oxide  being  greatly  desired  in  the 
slag.    A  selective  reduction  is  therefore  essential,  and  can  be  main-* 
tained  more  accurately  in  the  electric  furnace  than  in  fuel-fired 
furnaces,  as  the  carbon  added  serves  merely  as  a  reducer  and  is 
not  needed  for  supplying  heat. 

Careful  design  would  be  needed  in  such  furnaces  to  avoid  an  exces- 
sive loss  of  the  carbon  electrodes  through  the  action  of  slags  rich 
in  oxide  of  iron. 

Examples  of  the  electric  smelting  of  oxidized  ores  of  tin,  copper 


340  THE  ELECTRIC  FURNACE 

and  nickel  are  given  later.  Ores  containing  copper  in  the  metallic 
state  could  also  be  smelted  electrically;  no  carbon  being  added  to 
the  charge. 

(4)  Ores  containing  copper,  nickel  or  cobalt  as  sulphide  or  arsen- 
ide are  usually  smelted  in  a  blast-furnace,  often  without  prelimi- 
nary roasting,  and  the  sulphur  (or  arsenic)  and  the  iron,  with  which 
they  are  usually  associated,  are  oxidized  by  the  blast,  affording  a 
material  part  of  the  heat  needed  for  the  fusion  of  the  ore.  The 
product  of  this  operation  is  a  matte  (sulphides)  or  speiss  (arsenides) , 
containing  the  valuable  metals  and  part  of  the  iron,  and  a  slag 
(oxides)  containing  the  earthy  matter  and  most  of  the  iron. 

The  objects  of  the  operation  are  two:  (a)  to  fuse  the  ore  and 
so  to  separate  the  sulphides  or  arsenides  from  the  earthy  matter, 
and  (b)  to  oxidize  part  of  the  sulphur  (or  arsenic)  and  iron,  which 
would  otherwise  have  remained  with  the  copper  or  nickel,  so  as  to 
cause  the  sulphur  to  pass  off  as  a  gas,  and  the  iron  to  pass  as  oxide 
into  the  slag.  Such  an  operation  is  comparatively  foreign  to  ordi- 
nary electric- furnace  practice,  partly  because  an  air  blast  is  neces- 
sary for  oxidation,  and  partly  because  so  little  fuel  is  needed  in 
the  blast-furnace  that  there  seems  little  incentive  to  replace  it  by 
electrical  energy.  Little  has  been  done  in  this  direction,  but  the 
author  is  of  the  opinion  that  the  combination  of  the  blast-furnace 
with  the  electric  furnace  is  practicable,  and  would  secure  material 
advantages  in  some  cases,  provided  of  course  that  cheap  electrical 
power  could  be  obtained. 

Examples  of  the  electric  smelting  of  ores  of  copper,  nickel,  tin  and 
lead  will  now  be  given: 

Copper. — Sulphide  ores  of  copper,  in  which  the  sulphur  was  not 
much  more  than  was  required  to  make  a  high-grade  matte,  were 
smelted  in  a  Keller  furnace  at  Livet  in  France  in  the  year  1907, 
and  yielded  a  good  matte  and  a  clean  slag,  as  shown  by  the  following 
analyses:1 

Eight  metric  tons  of  the  charge  were  smelted  in  eight  hours, 
using  500  kw.,  the  average  current  being  4,750  amperes  at  119 
volts,  with  a  power  factor  of  0.9.  The  consumption  of  electrodes 
was  6  or  7  kg.  per  ton  of  charge.  The  consumption  of  electrical 
energy  amounted  to  500  kw.-hours  per  ton  of  charge,  and  a  furnace 
smelting  100  tons  daily  would  need  3,000  electrical  horse-power. 

1  M.  Vattier,  Report  of  Canadian  Commission  on  Electric  Smelting  in  Europe, 
Ottawa,  1904. 


THE  ELECTRIC  SMELTING  OF  COPPER 


341 


TABLE  XXII .— ELECTRIC  COPPER  SMELTING 


Ore 
Charge 

Slag 

Matte 

Copper.  .  , 

Per  cent. 
<;.  10 

Per  cent, 
o.  10 

Per  cent. 
47  •  QO 

Iron.  .  .            .  .            

28   <o 

32   so 

24    30 

Manganese 

7    64. 

8  23 

I    40 

Sulphur  
Phosphorus                  

4.12 

o  o"» 

0-57 
o  06 

22  .  96 
O    OI 

Silica 

23    7O 

27    2O 

o  80 

Alumina  
Lime 

4.00 
7    1O 

5.20 
0    GO 

0.50 

Magnesia  

O.  33 

O.  30 

Carbonic  acid.  . 

A.  31 

The  furnace  consisted  of  a  rectangular  chamber  6  ft.  by  3  ft. 
inside  and  3  ft.  high,  provided  with  two  carbon  electrodes,  12  in. 
square,  suspended  in  the  furnace  with  their  lower  ends  slightly  above 
the  slag  level  and  surrounded  with  the  ore  charge.  A  fore-hearth 
or  settling  chamber  4  ft.  by  2  ft.  and  2  ft.  high  was  heated  by  two 
electrodes  each  9  in.  square,  and  served  to  complete  the  separation 
of  the  matte  and  slag,  as  in  ordinary  practice.  It  need  not  be  added 
that  no  coke  or  other  fuel  was  used  in  this  operation  (as  would  be 
needed  in  iron  smelting),  the  process  being  mainly  a  simple  fusion  of 
the  ore  for  the  separation  of  the  sulphides,  as  matte,  from  the  oxides, 
which  form  slag.  Part  of  the  sulphur  in  the  charge  has  been 
removed  by  volatilization  or  oxidation. 

About  0.08  E.H.P.  years  of  electrical  energy,  and  12  Ib.  of  elec- 
trodes are  used  per  short  ton  of  charge.  Taking  the  power  at 
$15,  and  the  electrodes  at  3  cents,  gives  a  cost  of  $1.20  for  power  and 
36  cents  for  electrodes.  This  ore  could  be  smelted  in  a  large  water- 
jacketed  blast-furnace  with  12  per  cent,  to  15  per  cent,  of  coke. 
If  the  coke  can  be  obtained  at  a  reasonable  rate,  say  $6  a  ton,  the 
cost  for  fuel  will  be  about  80  cents,  and  the  blast-furnace  treatment 
will  clearly  be  cheaper  than  electric  smelting;  but  in  places  where 
coke  is  very  costly  and  especially  if  electrical  power  can  be  bought 
at  less  than  $15,  electric  smelting  may  become  the  better  method. 

In  comparing  the  two  methods,  it  should  be  remembered  that  the 
electric  furnace  will  in  general  produce  somewhat  cleaner  slags 
than  the  blast-furnace,  that  it  can  treat  difficulty  fusible  ores  more 
readily,  and  that  powdery  ores  will  not  be  so  objectionable  as  in  the 
blast-furnace. 

In  smelting  strongly  pyritic  ores,  it  is  necessary  to  oxidize  in  the 


342  THE  ELECTRIC  FURNACE 

furnace  a  large  proportion  of  the  sulphur  and  associated  iron,  in 
order  that  a  sufficiently  high-grade  copper-matte  can  be  obtained. 
This  is  accomplished  in  "Pyritic  Smelting"  by  the  use  of  a  large 
amount  of  air  and  a  small  amount  of  fuel  (about  5  per  cent,  of  the 
charge),  so  that  the  conditions  are  strongly  oxidizing  and  the  sulphur 
and  iron  are  largely  oxidized;  incidentally  furnishing  most  of  the 
heat  needed  for  the  reaction.  In  order  to  oxidize  the  sulphur  and 
iron,  the  fuel  in  the  charge  must  be  kept  very  low,  and  as  it  is 
practically  all  burnt  before  reaching  the  zone  of  fusion,  there  is 
danger  of  the  furnace  "freezing."  Electrical  smelting  would  seem 
particularly  suitable  for  such  an  operation.  A  smelting  shaft, 
provided  with  tuyeres  like  an  ordinary  blast-furnace,  would  termin- 
ate below  in  a  crucible,  heated  electrically,  where  the  half-melted 
materials  would  be  reduced  to  a  state  of  perfect  fusion.  Such  a 
furnace  could  be  started  with  fuel,  and  then  the  electric  current 
could  be  switched  on  and  the  fuel-supply  discontinued  or  greatly 
reduced.  In  such  a  furnace  the  ore  itself  would  supply  nearly  all 
the  heat;  the  small  proportion  furnished  electrically  merely  serving 
to  make  good  a  small  deficiency  of  heat  and  to  improve  the  general 
working  of  the  furnace.  The  electrodes  would  enter  the  furnace  at  a 
point  below  the  tuyeres,  so  as  to  be  protected  from  the  blast.  Dr. 
Heroult,1  has  patented  a  furnace  for  this  purpose,  in  which  the  elec- 
trode passes  down  through  the  charge,  but  is  protected  from  the 
air  blast  by  a  water-cooled  jacket.  The  lower  end  of  the  electrode, 
which  is  in  contact  with  the  charge,  is  below  the  tuyere  level. 

The  most  serious  consideration  in  designing  such  a  furnace  would 
be  to  keep  down  the  electrode  consumption,  which  would  probably 
be  excessive  if  carbon  electrodes  were  exposed  to  the  descending 
stream  of  oxidized  slag.2 

Nickel. — Oxidized  ores  of  nickel  have  been  smelted  electrically 
at  the  plant  of  the  Consolidated  Nickel  Company  at  Webster  in 
North  Carolina.3  The  ore  is  a  hydrous  silicate  of  magnesium 
and  nickel,  which  is  very  variable  in  composition,  and  generally 
contains  less  than  2  per  cent,  of  nickel.  The  ore  was  smelted  in  a 
simple  electric  furnace  with  the  addition  of  crushed  coke,  the  prod- 

^.L.  T.  Heroult,  U.  S.  patent  930,666,  1909,  Electrochem.  and  Met.  Ind., 
vii,  1909,  p.  407. 

2  The  use  of  electric  heat  in  the  copper  blast-furnace  and  reverberatory  fur- 
nace was  suggested  by  S.  B.  Ladd,  Met.  and  Chem.  Eng.,  viii,  1910,  p.  7. 

3W.  L.  Morrison,  "  Electric  Furnace  Treatment  of  Nickel  Ores  and  the  Devel- 
opment of  a  Commercial  Process,  "Trans.  Am.  Electrochem.  Soc.,  xx,  1911^.315. 


THE  ELECTRIC  SMELTING  OF  NICKEL 


343 


ucts  being  a  nickel-ferrosilicon  and  a  fusible  slag  of  remarkable  com- 
position, as  shown  in  the  following  table: 

TABLE  XXIII.— ELECTRIC  NICKEL  SMELTING 


Metal 
Per  cent. 

Slag 
Per  cen 

t. 

Nickel             13-14 

Silica, 

AQ—AC 

Lime 

3—  10 

Iron                 "N"?—  <\7 

Alumina 

2O—3O 

Magnesia  

it;—  20 

Silicon              24—27 

Ferrous  oxide 

O    "?—  2 

Nickel  oxide 

o  4.—  o  8 

Alkalies  

5-9 

FIG.  137. — Nickel  furnace. 

The  furnace  was  similar  to  the  Heroult  furnace  used  for  smelt- 
ing iron-ores  at  Sault  Ste.  Marie  (Fig.  78).  The  power  used  was 
about  170  kw.,  and  3.5  metric  tons  of  ore  were  smelted  per  24  hours, 
giving  an  energy  consumption  of  800  kw.-days  per  short  ton  of  ore. 

Mr.  Morrison  has  designed  a  shaft  furnace,  Fig.  137,  for  smelt- 


344  THE  ELECTRIC  FURNACE 

ing  nickel  ores,  although  he  does  not  recommend  it  for  use  with 
the  Carolina  ores,  which  are  very  powdery.  The  furnace  consists 
of  a  smelting  chamber,  C,  having  a  carbon  hearth,  ££,  which  forms 
one  electrode,  and  a  number  of  movable  electrodes,  E  E,  entering 
through  the  roof  and  connected  to  the  other  pole  of  the  electrical 
supply.  The  shaft  of  the  furnace  is  placed  at  one  side  of  the  smelt- 
ing chamber,  and  reciprocating  plungers  are  provided  for  moving 
the  charge  into  the  latter.  Each  electrode  is  provided  with  a  water- 
cooled  collar  which  serves  to  hold  it  and  to  supply  the  electric 
current.  A  supply  of  coke  is  admitted  close  to  the  electrode  at  D  in 
order  to  reduce  the  electrode-consumption,  and  tuyeres,  A  A,  sup- 
ply air  to  the  shaft  for  burning  the  combustible  gases  formed  in 
the  smelting  chamber.  The  author  has  illustrated  this  furnace, 
although  it  has  not  been  constructed,  as  it  typifies  the  combination 
of  a  blast-furnace  and  an  electric  smelting  furnace,  such  as  might 
be  used  for  smelting  pyritic  ore  without  fuel.  (See  Copper.) 

Tin. — The  electric  furnace  has  been  used  successfully  for  smelting 
the  tin  drosses  produced  in  a  tin-plate  works.1  The  operation  was 
carried  out  in  a  small  shaft  furnace,  20  in.  in  diameter  and  80  in.  high 
inside.  One  electrode  was  in  the  bottom  of  the  furnace,  and  the  other 
was  suspended  within  the  shaft.  Some  slag  was  first  melted  in  the 
furnace,  and  then  the  dross  was  added  with  enough  carbon  for  its 
reduction.  The  process  is  very  satisfactory,  as  there  is  little  loss  of 
tin  by  volatilization  or  in  the  slag,  and  the  reduced  metal  is  refined  in 
passing  through  the  slag. 

The  smelting  of  tin  ores  is  effected  either  in  a  small  blast-furnace,  or 
in  a  reverberatory  furnace.  The  losses  by  volatilization  are  heavy 
in  the  blast-furnace,  and  the  reduction  is  imperfect  in  the  reverbera- 
tory furnace.  In  view  of  the  high  price  of  tin,  it  would  seem  prob- 
able that  the  electric  furnace  could  be  employed  profitably  for  tin- 
smelting,  on  account  of  the  saving  of  metal  which  it  would  almost 
certainly  make. 

Lead. — This  metal  is  reduced  so  easily  from  its  ores  by  the  usual 
methods,  that  it  would  hardly  seem  likely  that  electric  smelting 
could  be  profitably  substituted.  Lead  ores  are  usually  smelted  in  the 
blast-furnace,  but  there  is  a  decided  loss  by  volatilization,  and  very 
rich  ores  cannot  be  smelted  in  that  way,  as  the  losses  would  be  too 
high.  They  are  therefore  often  smelted  in  a  reverberatory  furnace  by 
a  combined  process  of  roasting  and  reaction.  This  process  is  not 

1  R.  S.  Wile,  "Reduction  of  Tin  Dross  in  an  Electric  Furnace,"  Trans.  Am. 
Electrochem.  Soc.,  xviii,  1910,  p.  205. 


ELECTRIC  SMELTING  OF  TIN  AND  LEAD        345 

very  economical,  and  the  losses  by  volatilization  and  imperfect  re- 
duction are  so  high  that  it  may  pay,  where  electrical  power  is  cheap, 
to  smelt  such  ores  in  the  electric  furnace.  It  has  already  been  shown, 
in  connection  with  zinc  smelting,  that  the  electric  furnace  reduces 
lead  ores  very  effectively,  and  the  losses  by  volatilization  will  be 
small. 


CHAPTER  XIII 

MISCELLANEOUS  USES  OF  THE  ELECTRIC  FURNACE 
NITRIC  ACID  AND  NITRATES 

Although  nitrogen  is  an  inert  element,  and  will  not  combine  with 
oxygen,  under  ordinary  circumstances,  yet  at  the  high  temperature 
of  the  electric  arc  it  does  form  an  oxide.  This  presents  the  appar- 
ently curious  case  of  a  furnace  in  which  the  air  serves  for  fuel  as  well  as 
to  support  combustion.  It  is  hardly  correct,  however,  to  call  nitrogen 
a  fuel  because  its  oxidation  is  very  slow  and  partial  and  no  heat  is 
produced  by  the  reaction. 

The  fact  that  oxygen  and  nitrogen  would  combine  in  the  electric 
arc  was  discovered  by  Priestley,  and  in  1785  by  Cavendish.  It  was 
investigated  by  Crookes  in  1893  and  used  by  Rayleigh  in  1897  for  the 
separation  of  argon  from  the  air.  Recently  the  reaction  has  been 
utilized  for  the  manufacture  of  nitric  acid  and  nitrates.  A  number 
of  processes  for  this  purpose  were  patented  in  the  years  1895-96. 
The  first  process  to  be  tested  on  a  commercial  scale  was  that  of 
Bradley  andLovejoy,  which  was  tried  at  Niagara  Falls  in  1902,  but 
proved  unsuitable  for  commercial  purposes.  A  process  by  Kowalski 
and  Moscicki  was  also  tried  on  a  commercial  scale  in  Switzerland  in 
1903,  but  has  since  been  abandoned.  The  first  process  which  proved 
successful  on  a  large  scale  was  that  of  Birkeland  and  Eyde,  invented 
in  1903,  which  is  now  in  operation  in  Norway,  and  at  the  present 
time  the  processes  of  Pauling  and  Schonherr  are  also  in  commercial 
operation. 

Before  describing  these  in  detail  a  general  account  maybe  given: 
In  each  process  a  high-tension  electric  arc  is  used  and  air  is  forced 
through  or  over  the  arc.  At  the  high  temperature  of  the  arc  it  is 
probable  that  the  nitrogen  and  oxygen  in  the  air  partly  dissociate 
into  their  constituent  atoms  which  recombine  to  form  nitric  oxide. 


This  reaction  is  reversible  and  only  proceeds  to  a  definite  limit 
which  varies  with  the  temperature.     The  following  are  the  maxi- 

346 


NITRIC  ACID  AND  NITRATES  347 

mum  percentages  by  volume  of  nitric  oxide  which  can  be  formed: 

At  1,500°  C  ...........................   0.5  per  cent.  NO. 

At  2,000°  C  ...........................    i.o  per  cent.  NO. 

At  3,000°  C  ...........................   5.0  per  cent.  NO. 

These  percentages  have  been  determined  experimentally,  and  agree 
very  well  with  the  percentages  calculated  by  Nernst  from  theo- 
retical considerations.  It  will  be  seen  that  the  proportion  of  nitric 
oxide  increases  very  rapidly  with  the  temperature,  and  it  is,  there- 
fore, desirable  to  heat  the  air  as  strongly  as  possible  in  the  electric 
furnace.  As  the  reaction  is  reversible  the  nitric  oxide  tends  to  dis- 
sociate while  cooling  from  the  temperature  of  the  furnace,  and 
it  is  consequently  necessary  that  the  gases  after  being  heated  shall 
be  cooled  as  rapidly  as  possible  to  avoid  this  decomposition  of  the 
nitric  oxide.  In  practice  the  gases  leaving  the  furnace  contain  from 
i  per  cent,  to  2  per  cent,  of  nitric  oxide.  The  gases  leave  the  fur- 
nace at  a  temperature  of  about  900°  C.  to  1,000°  C.,  and  after  cool- 
ing to  600°  C.  the  nitric  oxide  begins  to  combine  with  oxygen  to 
yield  nitrogen  peroxide.  This  reaction  is  not  complete  until  the 
gases  have  cooled  to  130°  C.  and  even  at  this  temperature  the  reac- 
tion proceeds  slowly.  The  reaction  is  expressed  in  the  following 
equation: 


Actually,  however,  the  reactions  are  less  simple  than  this,  and  the 
gases  contain  various  oxides  of  nitrogen,  such  as 

NO,  N2O3,  NO2  and  N2O4. 

After  the  gases  have  been  cooled  to  about  the  ordinary  temperature 
they  are  allowed  to  react  with  water,  forming  nitric  and  nitrous  acids 
as  shown  by  the  equations: 

3N02+H2O  =  2HN03+NO 
N2O4+H2O  =  HN03+HN02 
N2O3+H2O  = 


The  nitrous  acid  decomposes  yielding  nitric  acid  and  nitric  oxide. 

=  HNO3+H2O+2NO 


Most  of  the  nitrous  gases  can  be  collected  by  means  of  water  as 
nitric  and  nitrous  acid,  but  a  part  escapes  after  this  treatment  and  can 
only  be  collected  in  a  solution  of  carbonate  of  soda  or  milk  of  lime. 


348  THE  ELECTRIC  FURNACE 

The  nitric  acid  is  collected  in  towers  where  the  gases  meet  a  descending 
stream  of  water;  by  using  a  number  of  towers,  in  series,  and  passing 
the  liquid  from  the  bottom  of  one  tower  down  the  next  in  such  a 
way  that  the  liquid  and  the  gases  move  in  opposite  directions 
through  the  series  of  towers,  nitric  acid  of  a  strength  of  30  per  cent. 
to  40  per  cent,  pure  acid  can  be  obtained.  This  acid  can  be  further 
enriched  to  at  least  50  per  cent,  by  evaporation  in  contact  with 
the  hot  gases.  The  concentrated  acid  can  be  sold,  or  the  dilute 
acid  from  the  towers  may  be  used  to  dissolve  limestone  for  the 
production  of  calcium  nitrate,  which  is  utilized  for  a  fertilizer. 

Returning  to  the  furnace,  the  general  principle  of  each  furnace 
is  to  spread  out  an  electric  arc  so  that  it  shall  fill  a  large  space,  to 
blow  air,  which  may  be  heated,  through  the  arc,  and  to  remove  and 
cool  the  air  as  rapidly  as  possible.  Alternating  current  is  almost 
universally  employed  for  the  arc.  In  order  to  maintain  a  steady 
arc  it  is  necessary  to  use  a  resistance  coil  or  an  inductance  coil  in 
series  with  the  arc.  The  inductance  coil  wastes  less  energy  than  a 
resistance  coil,  and  is,  therefore,  used,  although  it  necessarily  lowers 
the  power-factor  of  the  apparatus. 

Birkeland  and  Eyde  Furnace. — In  this  furnace  an  alternating- 


FIG.  138. — Birkeland-Eyde  furnace. 

current  arc  is  formed  between  the  ends  of  two  water-cooled  copper 
electrodes,  E  E,  as  shown  in  Fig.  138,  and  a  strong  magnetic  field  at 
right  angles  to  the  arc  causes  it  to  move  either  upward  or  downward. 
As  the  arc  is  deflected  it  becomes  longer,  its  ends  running  along  the 
electrodes,  while  the  arc  itself  keeps  a  roughly  semi-circular  form 
as  shown  by  the  dotted  lines  in  the  figure.  As  the  current  is  alternat- 


NITRIC  ACID  AND  NITRATES 


349 


ing,  and  the  magnetic  field  is  steady,  being  produced  by  a  direct- 
current  electro-magnet,  the  arc  is  deflected  alternately  upward  and 
downward,  and  the  result  is  a  large  disc  of  flame,  which  is  about 
6  ft.  in  diameter  This  disc-shaped  arc  is  enclosed  in  a  furnace 
made  of  fire-brick,  as  shown  in  Fig.  139,  and  air  is  blown  in  through 
the  openings  B  B  and  enters  the  arc  from  both  sides  by  a  number 
of  small  passages  through  the  brickwork.  After  traversing  the  arc, 
the  gases  enter  the  passage  A  A ,  from  which  they  are  removed  by 
the  pipe  D.  The  spreading  of  the  arc  may  be  assisted  by  the 
radial  movement  of  the  air  within  the  furnace.  The  brickwork  of 
the  furnace  is  contained  within  a  cast-iron  case,  which  is  made  in 
two  parts;  and  the  electrodes  are  separated  from  the  walls  of  the 


FIG.  139. — Birkeland-Eyde  furnace. 

furnace  by  the  insulators  7  /.  The  electrical  arrangement  is  shown 
diagrammatically  in  Fig.  138,  consisting  of  an  alternator  A  and  a 
reactance  coil  R. 

The  magnetic  field  is  produced  by  an  electromagnet,  consisting 
of  a  laminated  iron  core,  C  C,  (Fig.  139)  and  coils  of  wire,  F  F, 
which  are  supplied  with  direct  current. 

The  furnaces  are  made  in  various  sizes  from  200  to  2,000  kw.  A 
i,6oo-kw.  furnace  gives  a  flame  about  6  ft.  6  in.  in  diameter,  and 
takes  a  current  of  500  amperes  at  5,000  volts.  The  reactance  coil 
lowers  the  working  voltage  at  the  arc  to  about  3,500  volts,  thus 
giving  a  power-factor  of  about  70  per  cent.  Three-phase  5o-cycle 
current  is  supplied  at  10,000  volts  and  two  furnaces  are  connected 
in  series  so  that  the  voltage  for  each  furnace  is  5,000. 

The  temperature  of  the  arc  in  the  furnace  is  supposed  to  be  about 


350  THE  ELECTRIC  FURNACE 

3,000°  C.  but,  on  account  of  the  cold  air  entering  through  the  walls, 
the  fire-brick  of  the  furnace  is  not  heated  much  above  700°  C.  The 
brickwork  is  found  to  last  for  five  or  six  months,  and  the  electrodes 
need  replacing  after  three  or  four  weeks.  The  electrodes  are  made 
of  copper  tubing  and  their  ends,  which  are  renewable,  are  less  than 
i  cm.  apart.  They  very  seldom  need  regulating  and  the  furnace 
itself  gives  ample  warning  when  any  adjustment  is  needed  by  the 
roaring  of  the  arc. 

In  Fig.  138  it  will  be  noticed  that  the  arc  is  not  quite  concentric. 
This  is  caused  by  the  different  behavior  of  the  arc  at  the  positive 
and  the  negative  electrodes.  If  the  lines  of  magnetic  force  are 
passing  outward  at  right  angles  to  the  plane  of  the  figure,  and  if,  at 
any  moment,  the  right  hand  electrode  is  positive  (as  shown  by 
the  upper  — f-  signs)  the  arc  will  be  deflected  upward.  As  the  arc 
expands,  its  extremities  move  more  quickly  along  the  positive  than 
along  the  negative  electrode,  and  therefore  the  semicircle  of  flame 
is  shifted  a  little  to  the  right.  When  the  electric  polarity  has  been 
reversed  (as  shown  by  the  lower  -\ —  signs)  the  arc  is  forced  down- 
ward by  the  magnetic  field,  and  moving  more  rapidly  along  the 
positive  than  the  negative  electrode  is  shifted  to  the  left. 

The  gases  leaving  the  furnace  are  passed  through  a  boiler  for 
raising  steam,  then  through  a  system  of  aluminium  pipes,  cooled 
with  water,  and  then  into  oxidation  towers,  which  are  iron  cylinders 
lined  with  acid-proof  stone.  The  gases  next  pass  to  the  absorption 
towers,  which  are  built  of  granite  and  are  filled  with  broken  quartz, 
down  which  trickles  water  and  dilute  acid.  The  gases  finally  pass 
through  wooden  towers  where  they  meet  a  solution  of  soda  for 
absorbing  the  remainder  of  the  nitrous  gases.  The  absorption  is 
nearly  complete,  97  per  cent,  of  the  nitric  oxide  being  ultimately 
collected.  The  nitric  acid  from  the  towers  contains  about  30  per 
cent,  pure  acid  and  is  used  for  the  production  of  calcium  nitrate 
by  adding  it  to  limestone  in  granite  receptacles.  The  gases  leaving 
the  furnace  contain  from  i.o  to  1.5  per  cent,  of  nitric  oxide;  the 
amount  of  nitric  oxide  formed  is  probably  from  500  to  600  kg. 
of  pure  acid  per  kilowatt  year,  although  one  writer  has  stated  that 
900  kg.  are  formed.  The  amount  of  air  used  is  about  50  liters  per 
minute  for  each  kilowatt  employed.  This  would  correspond  to  a 
production  of  about  i  per  cent,  nitric  oxide  in  the  air.  For  a 
richer  product  less  air  would,  of  course,  be  used. 

The  number  of  furnaces  in  operation  is  about  36,  employing 
30,000  h.p.  An  additional  development  of  power  has  been  ar- 


h 

5 

'NITRIC  ACID  AND  NITRATES  351 

ranged  for  aURjukanfos  of  180,000  h.p.  for  this  process,  and  it  is 
expected  that  a  production  of  100,000  tons  per  annum  of  calcium 
nitrate  will  soon  be  effected. 

The  cost  of  electrical  power  at  Notodden  is  about  $4  or  $5 
per  horse-power  year,  corresponding  to  about  0.04  cents  per 
kilowatt  hour. 

REFERENCES  TO  THE  BIRKELAND  AND  EYDE  PROCESS 

Kr.  Birkeland,  "The  Oxidation  of  Atmospheric  Nitrogen  in  Electric  Arcs," 
Trans.  Faraday  Soc.,  vol.  ii,  1906,  p.  98. 

J.  Zenneck,  Met.  and  Chem.  Eng.,  vol.  ix,  1911,  p.  73. 

E.  Lamy,  Met.  and  Chem.  Eng.,  vol.  ix,  1911,  p.  100. 

J.  S.  Edstrom,  Am.  Electrochem.  Soc.,  vol.  vi,  1904,  part  II,  p.  17. 

E.  K.  Scott,  "The  Manufacture  of  Nitrates  from  the  Atmosphere,"  Jour. 
Roy.  Soc.  Arts.,  Ix,  1912,  p.  647. 

The  Pauling  Furnace. — In  this  furnace  the  electrical  flame  is 
spread  by  means  of  a  jet  of  air  instead  of  by  a  magnet.  The  arc 
is  formed  between  two  water-cooled  steel  electrodes,  shaped  like 
the  horns  of  a  lightning  arrester.  These  electrodes  may  be  curved 
or  may  be  straight  as  shown  in  Fig.  140.  The  arc  is  formed  between 
the  electrodes  at  their  nearest  point  and  is  forced  upward  by  a 
blast  of  heated  air  from  the  tuyere  T.  As  the  alternating  current 
reverses,  the  arc  dies  away,  and  a  new  arc  is  struck  and  is  again 
forced  up  by  the  air  blast;  in  this  way  a  continuous  flame  is  main- 
tained which  may  be  2  ft.  or  3  ft.  in  height.  The  electrodes  are 
necessarily  about  1.5  in.  apart,  in  order  to  allow  the  passage  of 
the  air.  This  distance  is  too  great  for  the  formation  of  the  arc, 
and,  therefore,  lighting  knives  are  employed  for  starting  the  arc; 
these  are  thin  blades  of  copper  which  pass  through  a  slot  in  each 
electrode  and  nearly  meet  in  the  middle  of  the  gap,  as  shown  in 
black  in  the  figure.  The  arc  is  formed  between  these  knives  and 
passes  upward  on  to  the  electrodes  themselves.  The  electrodes 
last  200  hours,  but  the  knives  need  renewing  after  about  20  hours 
and  are  regulated  as  they  burn  away.  The  furnaces  are  built  of 
fire-brick  about  3  ft.  3  in.X3  ft.  8  in.  and  10  ft.  high  and  contain 
two  of  these  arcs  in  adjoining  chambers.  One  furnace  uses  400 
kw.,  200  in  each  arc;  the  two  arcs  being  placed  in  series.  The  air 
which  enters  through  the  tuyere  T  is  preheated  in  a  set  of  pipes  by 
the  hot  gases  leaving  the  furnace.  This  preheating  increases  the 
efficiency  of  the  furnace  and  lessens  the  danger  of  putting  out  the 
arc  by  the  blast  of  air.  The  sudden  cooling  of  the  gases  after  leav- 


352 


THE  ELECTRIC  FURNACE 


ing  the  arc  is  effected  by  cold  air  which  enters  below  each  arc,  at 
Ay  and  also  in  the  passage  B  between  the  two  arcs.  If  fresh  air 
were  used  for  this  purpose  the  effect  would  be  to  dilute  the  nitric 
oxide  in  the  resulting  gas;  so  to  avoid  this,  some  of  the  gases  from 
the  furnace,  after  being  cooled,  are  returned  and  used  for  the  cool- 
ing currents  already  referred  to.  The  amount  of  air  used,  apart 
from  the  cooling  gases,  is  about  10  cu.  m.  per  minute,  correspond- 
ing to  25  liters  per  minute  for  each  kilowatt. 


FIG.  140. — Pauling  furnace. 

The  gases  from  the  Pauling  furnace  contain  from  1.15  to  1.50 
per  cent,  of  nitric  oxide;  they  leave  the  furnace  at  900°  C.  to  1,000° 
C.  and  pass  first  through  the  preheater  where  some  of  their  heat 
is  used  for  heating  the  incoming  air;  they  then  pass  through  a  cool- 
ing tower  filled  with  brickwork.  When  the  bricks  have  become 
heated  the  gases  are  diverted  to  another  tower  while  the  first  is 
being  cooled  by  a  current  of  cold  air  drawn  through  it  by  means  of 
a  chimney.  After  leaving  the  cooling  tower  the  gases  enter  a  large 


NITRIC  ACID  AND  NITRATES  353 

tower  of  reinforced  concrete,  known  as  the  oxidation  tower,  where 
the  nitrous  oxide  combines  with  oxygen  to  form  nitrogen  peroxide. 
They  then  pass  through  a  number  of  absorption  towers  for  the 
production  of  nitric  acid,  and  finally  they  are  led  through  some 
" nitrite  towers"  where  the  residual  gases  are  absorbed  by  a  solu- 
tion of  sodium  carbonate.  The  inventors  guarantee  a  production 
of  60  grm.  of  pure  acid  per  kilowatt  hour,  which  corresponds  to  a 
little  more  than  500  kg.  per  kilowatt  year.  The  plant  is  stated  to 
cost  not  more  than  $24  per  kilowatt. 

The  first  plant  on  the  Pauling  system  was  erected  at  Patsch,  near 
Innsbruck  in  the  Tyrol,  where  24  furnaces  are  operated  with  15,000 
h.p.  Two  other  plants  have  since  been  erected,  one  at  La  Roche- 
de-Rame  in  France,  and  one  in  Italy;  each  of  these  uses  10,000  h.p. 

In  the  plant  at  Patsch  there  are  24  furnaces,  of  400  kw.  each,  and 
in  the  French  plant  there  are  nine  furnaces  of  600  kw.  each. 

REFERENCES  TO  THE  PAULING  PROCESS 

F.  Russ,  "The  Innsbruck  Plant  for  Nitric  Acid  from  Air,"  Electrochem.  and 
Met.  Ind.,  vol.  vii,  1909,  p.  430. 

J.  Zenneck,  Met.  and  Chem.  Eng.,  vol.  ix,  1911,  p.  73. 

E.  Lamy,  Met.  and  Chem.  Eng.,  vol.  ix,  1911,  p.  102. 

J.  Vanderpol,  Met.  and  Chem.  Eng.,  vol.  ix,  1911,  p.  196. 

E.  K.  Scott,  "The  Manufacture  of  Nitrates  from  the  Atmosphere,"  Jour. 
Roy.  Soc.  Arts.,  Ix,  191 2,  p.  656. 

The  Schonherr  Furnace. — In  this  furnace,  which  was  invented 
by  Dr.  Schonherr,  and  is  operated  by  the  Badische  Analin  and  Soda- 
Fabrik,  a  long  electric  arc  is  formed  passing  along  the  axis  of  an 
iron  tube.  The  arc  extends  from  an  insulated  electrode  at  one  end 
of  the  tube  and  passes  to  the  wall  of  the  tube  near  the  other  end. 
The  arc  is  maintained  in  this  position  by  a  whirling  current  of  air 
passing  up  the  tube,  and  this  air  becomes  heated  and  forms  nitric 
oxide.  The  upper  end  of  the  tube,  where  the  arc  strikes,  being 
water-cooled,  serves  also  for  the  rapid  cooling  of  the  air  which  has 
passed  through  the  arc,  and  most  of  the  energy  supplied  to  the  fur- 
nace is  ultimately  wasted  in  heating  this  water.  In  the  apparatus, 
as  shown  in  Fig.  141,  it  will  be  seen  that  the  air  enters  the  furnace 
near  the  lower  end  and  passes  up  and  down  before  entering  the 
central  tube  which  contains  the  arc;  in  this  way  it  is  preheated  to  a 
certain  extent  by  the  waste  heat  of  the  issuing  gases,  and  also  serves 
to  prevent  the  over-heating  of  the  central  iron  pipe.  The  whirling 
motion  is  given  to  the  air  by  the  direction  of  the  holes  through 

23 


354 


THE  ELECTRIC  FURNACE 


which  it  enters  the  central  pipe  at  its  lower  end.  The  arc  strikes 
in  the  first  place  between  the  electrode  and  the  lower  end  of  the 
pipe,  it  is  immediately  carried  up  the  pipe  by  the  current  of  air, 

and  remains  in  a  steady  condition 
in  the  axis  of  this  pipe.  The  arc 
so  produced,  in  the  case  of  a 
6oo-h.p.  furnace,  is  about  16  ft. 
long.  A  i,oooh.p.  arc  is  as  much 
as  23  ft.  or  even  26  ft.  long.  The 
voltage  used  is  4,000  or  5,000  volts. 
The  gas  produced  by  this  process 
contains  as  much  as  2  per  cent,  of 
nitric  oxide,  and  after  cooling  the 
nitrous  gases  are  absorbed  by  milk 
of  lime  with  the  formation  of  calcic 
nitrate  and  nitrite,  which  are  util- 
ized for  fertilizing  purposes. 

REFERENCES  TO  THE  SCHONHERR 
PROCESS 

Dr.  Schonherr,  "The  Manufacture  of 
Air-saltpeter  by  the  Process  of  the  Bad- 
ische  Analin  and  Soda-Fabrik."  Trans. 
Am.  Electrochem.  Soc.,  vol.  xvi,  1909,  p. 

131- 

J.  Zenneck,  Met.  and  Chem.  Eng., 
vol.  ix,  1911,  p.  73. 

E.  Lamy,  Met.  and  Chem.  Eng.,  vol. 
ix,  1911,  p.  ici. 

E.  K.  Scott,  loc.  cit.,  p.  648. 

Calcium  Cyanamide.1  —  This 
compound,  CaCN2,  which  has  been 
described  under  "Uses  of  Calcium 
Carbide"  in  Chapter  XI,  may  be 
referred  to  at  this  place  as  another 
product  of  the  fixation  of  atmos- 
pheric nitrogen.  It  is  formed  by 
heating  calcium  carbide  in  nitrogen  in  an  iron  retort  to  a  temperature 
of  about  1,000°  C.  and  is  thus  indirectly  a  product  of  the  electric  fur- 
nace. It  is  employed  as  a  fertilizer  to  furnish  nitrogen  to  the  soil. 

*E.  Lamy,  "The  Industrial  Fixation  of  Atmospheric  Nitrogen,"  Abstract  in 
Met.  and  Chem.  Engineering,  ix,  1911,  p.  99. 


FIG.  141. — Schonherr  furnace. 


FUSED  QUARTZ 


355 


Fused  Quartz. — Silica,  the  oxide  of  silicon,  is  a  valuable  re- 
fractory material  for  lining  metallurgical  furnaces,  and  its  use  for 
this  purpose  has  been  described  in  Chapter  IV.  Although  very 
refractory,  silica  or  quartz  can  be  fused,  and  it  then  possesses 
valuable  properties,  and  has  been  used  for  some  time  in  the  con- 
struction of  scientific  apparatus.  As  an  example,  the  well-known 
quartz  filaments  of  C.  V.  Boys  may  be  mentioned.  These  are 
made  by  melting  the»quartz  in  the  oxyhydrogen  blow-pipe.  Recently 
it  has  been  found  possible  to  fuse  quartz  in  the  electric  furnace 
and  to  make  tubes,  crucibles,  dishes  and  other  articles  out  of  the 
fused  quartz.  This  material  scarcely  expands  at  all  when  heated, 


FIG.  142. — Furnace  for  fusing  silica. 

its  coefficient  of  expansion  being  only  one-twentieth  of  that  of  glass. 
In  consequence  of  this  it  is  possible  to  plunge  a  red-hot  article  made 
of  fused  quartz  into  cold  water  without  cracking  it.  Fused  quartz 
is  a  transparent  glass,  but  "  Electroquartz "  or  silica  melted  in  the 
electric  furnace  has  a  milky  white  color.1 

Fig.  142  shows  an  electric  furnace  for  the  manufacture  of  this 
material  by  the  process  of  Bottomly  and  Paget.2  The  furnace 
consists  of  an  iron  box  mounted  on  trunnions,  and  provided  with 
electrode  holders  at  the  ends.  Between  the  electrodes,  EE,  is 
supported  a  graphite  rod  of  smaller  diameter  which  serves  as  the 
resistor.  Around  this  rod  the  box  is  filled  with  silicious  sand;  the 
lid  being  removed  for  this  purpose.  The  sand  is  very  pure  (from 

Electrochemical  and  Metallurgical  Industry,  vol.  iv,  pp.  369  and  502;  vol. 
v,  pp.  67  and  107. 

2  A.  Pohl,  Zeit.  fur  angewandte  Chemie,  vol.  xxxvi,  1912.  p.  1845. 


356  THE  ELECTRIC  FURNACE 

99.6  per  cent,  to  99.8  per  cent,  silica)  and  the  grains  are  very  fine 
and  of  uniform  size.  A  current  of  1,000  amperes  at  15  volts  is 
passed  through  the  rod;  heating  the  surrounding  sand  to  a  temper- 
ature of  1,700°  or  1,750°  C.,  at  which  it  melts.  The  tube  of  fused 
silica,  FF,  made  in  this  manner  does  not  adhere  to  the  graphite  rod, 
but  is  distended,  as  shown  in  the  figure,  by  carbon  monoxide 
resulting  from  the  reaction  of  the  carbon  of  the  rod  with  a  little  of 
the  silica.  A  tube  formed  in  this  way  is  usually  rough  on  the 
outside,  due  to  adherent  sand,  and  to  prevent  this  a  graphite  tube 
can  be  imbedded  in  the  sand,  as  shown  in  the  figure,  so  that  the 
silica  tube  will  be  formed  in  contact  with  its  inner  surface. 

If  the  box  were  kept  in  a  horizontal  position,  the  silica  tube  would 
be  irregular  in  cross-section,  due  to  its  own  weight,  and  to  avoid 
this  the  whole  furnace  can  be  rotated  until  the  axis  is  vertical. 

When  it  is  judged  that  the  tube  of  fused  silica  is  sufficiently  thick, 
the  current  is  cut  off,  the  box  rapidly  opened,  the  pasty  tube  of 
silica  from  which  the  graphite  rod  has  been  removed  is  seized  by  one 
end  in  a  suitable  holder,  the  other  end  being  closed,  and  blown  by 
compressed  air  into  the  desired  form.  The  temperature  of  the  tube 
falls  considerably  during  this  manipulation,  but  the  silica  can  be 
blown  as  long  as  it  remains  above  1,500°  C.  The  true  melting-point 
of  silica  has  been  given  as  1,630°  C.,  but  it  does  not  become  really 
liquid  until  about  1,750°,  and  there  is  a  considerable  range  of  temper- 
ature below  1,630°  in  which  the  "fused  silica"  remains  pasty.  Arti- 
cles of  fused  quartz  or  fused  silica  are  chiefly  valuable  on  account  of 
their  refractory  qualities — 'being  far  more  refractory  than  glass — • 
and  on  account  of  their  freedom  from  expansion  when  heated. 
The  following  figures  are  of  interest  in  this  connection: 

COEFFICIENT  OF  LINEAR  EXPANSION 

Quartz  (along  principal  axis) o. 000,007,81 

Rock  crystal  (across  principal  axis) o. 000,014,19 

Fused  quartz o. 000,000,59 

Glass 0.000,008,83 

Porcelain 0.000,003,14 

Glass. — Although  glass  can  easily  be  melted  in  furnaces  fired  by 
gas,  the  cleanliness  and  convenience  of  electrical  heating  have  led 
to  its  use  in  the  manufacture  of  glass,  and  a  large  number  of  fur- 
naces have  been  devised  for  this  purpose.  In  these  furnaces  the 
glass-forming  materials  are  usually  heated  and  melted  by  means 
of  electric  arcs,  but  resistance  heating  is  sometimes  employed. 


ALUNDUM  357 

Trouble  has  been  experienced  in  regard  to  the  contamination  and 
discoloration  of  the  glass  with  the  particles  from  the  electrodes. 

Alundum. — This  is  an  artificial  corundum  or  emery  made  by 
fusing  bauxite  in  an  electric  furnace,  and  allowing  it  to  cool  slowly, 
thus  forming  a  hard  and  tough  crystalline  mass  which  is  broken  up 
and  used  as  an  abrasive.  The  process  was  invented  by  C.  B. 
Jacobs,1  and  the  material  is  manufactured  at  Niagara  Falls  by  the 
Norton  Company.2 

Bauxite  is  a  hydrated  form  of  alumina  and  contains  silica,  iron, 
titanium,  etc.,  as  impurities.  It  is  prepared  by  calcining  in  a  rotary 
kiln  to  expel  the  combined  water.  Bauxite  is  a  very  refractory 
material,  melting  at  about  2,000°  C.  and  it  could  not  be  melted 
commercially  without  the  aid  of  the  electric  furnace. 

A  small  amount  (5  per  cent.)  of  carbon  may  be  added  in  order  to 
reduce  to  the  metallic  state  the  iron,  silicon  and  titanium  contained 
in  the  bauxite;  thus  leaving  the  alundum  as  nearly  pure  fused 
alumina.  The  reduced  metals  are  in  nodules  and  can  be  separated 
mechanically  from  the  product. 

The  resulting  alundum  becomes  crystalline  during  its  slow  cool- 
ing and  is  crushed  and  sized  for  use  as  an  abrasive. 

When  it  is  desired  to  cement  these  grains  into  a  wheel  or  other  arti- 
cle for  grinding  purposes,  they  are  first  roasted,  in  order  to  oxidize 
any  metallic  particles  or  carbide,  arid  are  then  mixed  with  about 
i  per  cent,  of  fire-clay,  wetted,  pressed  into  shape  and  fired. 

Another  process  consists  in  melting  a  pure  form  of  bauxite  in  an 
electric  furnace  with  graphite  electrodes  (instead  of  carbon  elec- 
trodes) so  as  to  avoid,  as  far  as  possible,  any  reducing  action  and 
any  formation  of  aluminium  carbide.  The  crushed  product  is 
roasted  in  order  to  destroy  any  carbide,  and  the  particles  are  graded 
and  molded  with  a  bond  of  equal  parts  of  ball-clay  and  feldspar, 
using  one  part  of  the  bond  to  four  parts  of  the  alundum  grains. 

An  electric  furnace  suitable  for  the  manufacture  of  alundum  has 
been  patented  by  A.  C.  Higgins.3 

This  furnace,  shown  in  Fig.  143,  consists  of  an  iron  shell  S,  pro- 
vided with  an  external  water-cooling  system,  and  fitting  into  a 
ring  R  which  forms  part  of  the  base  of  the  furnace.  Two  electric 
arcs  (operated  in  series)  are  maintained  between  the  vertical  elec- 

1  C.  B.  Jacobs,  U.  S.  patent    659,926,    Electrochemical  Industry,   vol.  iii, 
1905,  p.  406. 

2  Electrochemical  Industry,  vol.  i,  1902,  p.  15. 

3  A.  C.  Higgins,  U.  S.  patent  775,654,  Electrochem.  Ind.,  iii,  p.  30. 


358 


THE  ELECTRIC  FURNACE 


trodes,  EE,  and  the  molten  alundum  A .  A  hood,  H,  placed  over 
the  furnace  serves  to  confine  the  dust  and  gases,  which  are  led  away 
by  a  flue,  F;  an  opening  for  charging  being  also  provided.  In 
starting  the  furnace ,  electric  arcs  are  established  between  the  elec- 
trodes and  the  carbon  lining  of  the  furnace  bottom;  calcined  bauxite 
is  then  fed  in  around  the  arcs  and  melts,  ultimately  carrying  the 


FIG.  143. — Alundum  furnace. 

electric  current  between  the  two  arcs.  As  the  amount  of  fused 
alundum  increases  the  electrodes  would  need  raising,  but  in  this 
case  the  same  purpose  is  served  by  lowering  the  furnace  which  is 
supported  on  a  hydraulic  ram.  As  the  furnace  fills  up,  the  lower 
part  of  the  alundum  solidifies,  as  shown  in  the  drawing.  When  the 
furnace  is  full,  the  shell  with  its  contents  can  be  lifted  from  the  base, 
R,  and  another  shell  placed  in  position  ready  for  a  fresh  charge. 


PHOSPHORUS  359 

The  shell  is  somewhat  conical  to  permit  the  easy  removal  of  the 
solidified  alundum. 

The  furnace  described  above  is  employed  by  the  Norton  Com- 
pany for  the  manufacture  of  alundum.  The  crucible  is  4  ft.  in  diam- 
eter and  5  ft.  high,  and  the  electrodes  are  each  4  in.  by  12  in. 
in  cross-section.  The  ingot  formed  weighs  about  2.5  tons.1  Mr. 
Higgins  has  invented  a  larger  furnace,2  of  similar  design  which  has 
four  electrodes. 

The  production  of  alundum  in  the  year  1911  was  about  5,000  tons. 

Phosphorus. — This  is  obtained  by  heating  phosphoric  acid  with 
carbon,  or  bone-ash  or  mineral  phosphates  with  carbon  and  silica. 
In  this  way  the  phosphorus  is  liberated  from  the  compound  contain- 
ing it,  and  is  distilled  and  condensed  outside  the  furnace.  As  this 
operation  must  be  carried  on  in  the  absence  of  air,  the  electric  furnace 
is  particularly  suitable,  and  has  practically  replaced  all  others. 

The  old  method  of  making  phosphorus  consisted  in  heating 
bone-ash  (calcium  phosphate)  with  sulphuric  acid,  thus  obtaining 
phosphoric  acid  or  calcium  metaphosphate.  This  was  mixed  with 
charcoal  and  heated  in  clay  retorts,  phosphorus  being  liberated  as  a 
vapor  and  condensed  under  water. 

Bone-ash  or  natural  calcium  phosphate  can  be  smelted  directly 
(without  the  sulphuric-acid  treatment)  to  yield  phosphorus,  if  sil- 
ica is  added  to  combine  with  the  lime,  as  well  as  charcoal  to  reduce 
the  oxide  of  phosphorus,  but  this  process  could  not  be  carried  out 
until  the  introduction  of  the  electric  furnace,  as  the  temperature 
required  was  higher  than  could  conveniently  be  obtained  in  a  retort. 
At  the  present  time,  bone-ash,  or  the  minerals  rock-phosphate, 
apatite,  or  wavelite,  are  mixed  with  silica  and  charcoal,  and  heated 
in  an  electric  furnace,  yielding  phosphorus  vapor,  carbon  monoxide 
and  a  liquid  slag  of  calcium  silicate.  In  some  cases,  however,  the 
sulphuric  acid  process  is  still  employed  as  a  preparatory  step. 

A  suitable  electric  furnace  is  that  invented  by  G.  C.  Landis,3 
and  shown  in  Fig.  144.  It  consistsof  an  iron  box  lined  with  vitrified 
bricks  and  having  an  inner  lining  of  carbon  blocks  which  form  one 
electrode;  the  other  electrode,  E,  being  vertical.  The  charge  of 
phosphate,  carbon  and  silica  is  introduced  by  means  of  the  hopper 
H  and  inclined  tube  7.  The  phosphorus  vapor  and  accompanying 

1  Mineral  Industry,  vol.  xx,  1911,  p.  31. 

2  Electrochem.  and  Metall.  Ind.,  vii,  1909,  p.  223. 

3  G.  C.  Landis,  U.  S.  patent  842,090,  1907,  assigned  to  American  Phosphorus 
Co.      Electrochem.  and  Metall.  Ind.,  v,  1907,  p.  55. 


360 


THE  ELECTRIC  FURNACE 


gases  pass  out  by  the  pipe  O  to  suitable  condensers,  and  the  fused 
slag  is  tapped  out  below.  The  furnace  is  made  gas-tight  with  the 
aid  of  water-seals  for  the  cover  and  for  the  gland  admitting  the 
electrode;  the  latter  being  insulated  from  the  furnace  body  at  the 
point  F.  The  vitrified  bricks  lining  the  furnace  are  laid  in  a  non- 
absorbent  mortar  such  as  a  mixture  of  silicate  of  soda  and  powdered 
asbestos;  this  being  intended  to  prevent  absorption  of  the  phosphorus. 
The  carbon  lining  is  composed  of  t.wo  layers  of  blocks,  so  that  the 
inner  set  of  blocks  can  be  replaced,  when  worn  away,  without  dis- 


FIG.  144. — Phosphorus  furnace. 


turbing  the  outer  layer.  The  inner  tapping  hole  for  the  molten 
slag  is  plugged  with  a  piece  of  wood,  without  any  luting;  an  outer 
plug,  which  is  luted,  prevents  access  of  air  to  the  inner  wooden  plug. 
Two  sets  of  tapping  holes  are  provided  so  that  one  set  can  be  used 
when  the  other  has  become  worn  out. 

About  80  per  cent,  or  90  per  cent,  of  the  phosphorus  in  the  rock  is 
extracted  by  this  process,  and  the  yield  has  been  stated  as  3.3  Ib.  of 
phosphorus  per  horse-power  day.  The  output  of  phosphorus  in 


CARBON  BISULPHIDE 


361 


1906  has  been  estimated  at  from  1,000  tons  to  3,000  tons;  the  price 
ranging  from  45  cents  to  70  cents  per  pound. 1 

Carbon  Bisulphide. — This  important  compound  of  carbon  and 
sulphur  is  made  by  passing  sulphur 
vapor  over  red-hot  charcoal. 

Until  recently  the  reaction  was  ef- 
fected in  retorts  of  iron  or  clay  heated 
externally  by  a  coal  fire,  but  now  it 
is  made  largely,  if  not  entirely,  in 
electric  furnaces. 

A  furnace  for  this  purpose  was  in- 
vented by  Mr.  E.  R.  Taylor,2  of  Penn 
Yan,  N.  Y.,  who  thus  revolutionized 
this  industry  in  the  United  States. 
This  furnace,  shown  in  Fig.  145  is 
somewhat  complicated  in  design,  but 
consists  generally  of  a  heating  cham- 
ber provided  with  four  horizontal  car- 
bon electrodes  and  surmounted  by  a 
tall  shaft  containing  the  charcoal,  C, 
to  be  treated.  This  shaft  has  double 
walls,  and  the  annular  space  between 
them  is  filled,  through  the  hopper,  //, 
with  sulphur  which  continually  melts 
with  the  heat  of  the  furnace,  and  runs 
down  into  the  heating  chamber. 

Sulphur  is  also  introduced  through 
four  hoppers,  H  H,  one  above  each 
electrode  holder,  E  E,  and  through 
other  hoppers  supplying  the  annular 
chambers  shown  in  the  plan.  This 

sulphur  also  is  melted  by  heat  that  would  otherwise  be  wasted,  and 
flows  into  the  heating  chamber.  When  the  furnace  is  in  regular 
operation,  molten  sulphur  lies  in  the  bottom,  at  S,  and  vaporizes, 

1G.  W.  Stose.  "The  Production  of  Phosphate  Rock  and  Phosphorus  in  1906." 
U.  S.  Geol.  Survey;  Abstract  in  Electrochem.  and  Metall.  Ind.,  v,  1907,  p.  407. 

2E.  R.  Taylor,  U.  S.  patent  706,  128,  1902.  Electrochem.  Ind.,  i,  1902,  pp.  60, 
63,  and  76.  "The  Manufacture  of  Bisulphide  of  Carbon  in  the  Electric  Furnace," 
Trans.  Am.  Electrochem  Soc.,  i,  1902,  p.  115.  "Making  Bisulphide  of  Carbon 
in  an  Electric  Furnace."  The  Electrical  Age,  New  York,  October,  1902.  Jour. 
Franklin  Inst.,  Feb.,  1908,  p.  141.  "The  Manufacture  of  Carbon  Bisulphide." 
Jour.  Indust.  and  Eng.  Chem.,  iv,  Aug.,  1912. 


Section  A-B 


FIG.  145. — Carbon  bisulphide 
furnace. 


362  THE  ELECTRIC  FURNACE 

passing  up  through  the  heated  charcoal  which  lies  between  and 
above  the  electrodes.  The  general  temperature  must  be  low  in 
comparison  with  that  in  most  electric  furnaces.  Sulphur  melts  at 
115°  C.  and  boils  at  445°  C.,  so  that  the  molten  sulphur  must  be 
at  intermediate  temperatures,  while  the  charcoal  itself  will  be  de- 
cidedly hotter,  say  800°  or  1,000°  C.  in  the  middle  of  the  heating 
chamber. 

The  formation  of  carbon  bisulphide  according  to  the  equation 

C+S2  =  CS2 

is  stated  to  take  place  at  a  red  heat  and  less  completely  at  a  bright 
red  heat.  It  seems  probable  that  the  charcoal  in  the  vicinity  of  the 
electrodes  may  be  above  the  temperature  that  is  most  suitable  for 
the  reaction,  but  as  the  sulphur  vapor  passes  up  the  shaft,  it  will 
meet  charcoal  at  lower  temperatures,  and  will  be  largely,  if  not 
entirely,  converted  into  carbon  bisulphide. 

Charcoal  contains  about  3  per  cent,  of  ash  which  cannot  take 
part  in  the  above  reaction,  and  must  remain  in  the  furnace.  This 
ash  may  become  fused  in  the  hottest  zone  between  the  electrodes, 
but  owing  to  the  low  temperature  of  the  molten  sulphur  the  ash 
must  solidify  and  cannot  be  tapped  molten  from  the  furnace.  It 
will  therefore  be  left  in  the  furnace,  and  is  cleaned  out  about  once 
a  year,  by  which  time  several  tons  must  have  accumulated. l 

The  furnace  is  41  ft.  high  and  16  ft.  in  diameter,  built  of  iron  so 
as  to  be  gas-tight,  and  lined  with  brickwork  to  retain  the  heat. 
It  is  operated  with  two-phase  current  of  about  4,000  amperes  at 
from  40  to  60  volts.  This  is  supplied  directly  from  two  generators 
of  330 '  kw.  each,  run  by  water-power  supplemented  by  steam- 
power.  Each  electrode  is  20  in.  square  and  48  in.  long,  and  is  com- 
posed of  25  rods  of  carbon,  each  4  in.  square.  Above  each  elec- 
trode is  an  opening,  G,  through  which  broken  carbon  is  fed.  This 
serves  as  a  resistor  to  carry  the  current  between  the  electrodes,  as 
charcoal  itself  is  a  very  poor  conductor,  and  it  also  protects  the 
electrodes,  which  are  found  to  last  for  at  least  a  year.  The  output 
was  stated  in  1908  to  be  14,000  Ib.  per  day,  but  Mr.  Taylor  has 
recently  informed  the  author2  that  he  has  made  carbon  bisul- 

1  Mr.  Taylor  states  that  1,500,000  Ib.  to  2,000,000  Ib.  of  CSu  have  been  made 
in  a  furnace  without  cleaning  out;  these  figures  would  correspond  to  7,000  Ib. 
and  10,000  Ib.  of  charcoal  ash. 

8  E.  R.  Taylor,  Letter  of  Nov.  2,  1912.  The  author  is  indebted  to  Mr.  Taylor 
for  some  of  the  information  contained  in  this  account. 


MONOX  363 

phide  in  this  furnace  at  the  rate  of  17,000  Ib.  per  24  hours,  using 
4,000  amperes  at  50  to  80  volts,  and  that  he  is  now  constructing  a 
furnace  56  ft.  high  and  18  ft.  in  diameter,  which  will  be  capable  of 
a  still  greater  output. 

Carbon  bisulphide  is  a  clear  colorless  liquid  which  is  very  vola- 
tile and  has  an  unpleasant  smell.  Its  vapor  is  more  than  twice 
as  heavy  as  air,  and  as  it  is  also  poisonous,  carbon  bisulphide  is 
used  in  large  quantities  for  killing  rats,  mice,  gophers  and  other 
vermin.  Bisulphide  of  carbon  is  largely  used  in  the  arts  as  a  sol- 
vent for  oil,  gutta  percha,  etc.,  and  has  recently  been  employed  in 
making  artificial  silk  from  the  fiber  of  spruce  wood. 

Monox.  —  This  is  an  electric-furnace  product,  discovered  by 
Dr.  Potter,1  containing  silicon,  monoxide,  SiO,  together  with 
amorphous  silicon  and  silica. 

When  silica  is  heated  with  carbon  in  an  electric  furnace,  the 
first  product  of  reduction  is  apparently  silicon  monoxide. 

Si02+C  =  CO+SiO 

At  the  temperature  of  the  furnace,  silicon  monoxide  is  a  gas  and 
tends  to  escape,  but  if  kept  in  by  the  superincumbent  charge,  it  is 
further  reduced  to  silicon  which  condenses  and  collects  in  the 
molten  state  as  in  the  silicon  furnace. 


If  there  is  little  charge  above  the  zone  of  reaction,  the  silicon  mon- 
oxide escapes  with  the  carbon  monoxide,  and  burns  with  a  bril- 
liant flame  yielding  clouds  of  silica.  If,  however,  the  gases  are  al- 
lowed to  escape  into  a  large  container  free  from  air  and  preferably 
under  reduced  pressure,  they  do  not  burn,  and  the  silicon  monoxide 
and  other  vapors  condense  in  the  form  of  a  fine  brown  powder 
which  has  been  termed  "monox.  " 

"  Monox"  has  been  shown  to  consist  largely  of  silicon  monoxide, 
but  it  also  contains  amorphous  silicon  and  silica.  It  is  extremely 
light,  a  cubic  foot  of  the  loose  powder  weighing  only  2.5  Ib., 
although  its  true  density  is  2.24,  that  is  140  Ib.  per  cubic  foot. 

Monox  burns  easily,  without  flame,  when  fresh,  but  after  stand- 
ing it  becomes  more  inert  owing  to  a  superficial  oxidation  of  the 
particles. 

xDr.  H.  N.  Potter,  Trans.  Am.  Electrochem.  Soc.,  xii,  1907;  "Silicon  Mon- 
oxide," p.  191;  "Monox,"  p.  215.  "The  Production  of  Monox  a  New  Electric- 
furnace  Product,"  p.  223. 


364 


THE  ELECTRIC  FURNACE 


Monox  possesses  many  interesting  and  useful  properties  and  in 
particular  has  been  found  to  make  a  very  good  paint,  when  mixed 
with  oil.  It  can  also  be  employed  as  an  ingredient  of  printers' 
ink,  as  it  improves  the  " laying"  property  of  the  ink. 

A  furnace  used  by  Dr.  Potter  for  the  production  of  monox  is 
shown  diagrammatically  in  Fig.  I46,1  and  consists  of  a  cylindrical 
cast-iron  drum  4  ft.  in  diameter,  lined  with  7  in.  of  fire-brick,  and 
provided  with  two  horizontal  electrodes.  The  electrodes,  which 


FIG.  146. — Monox  furnace. 


are  4  in.  square,  are  made  of  carbon  or  Acheson  graphite  and 
are  provided  with  water-cooled  electrode  holders.  As  the  furnace 
must  be  air-tight,  these  holders  are  contained  in  cast-iron  terminal 
boxes,  which  are  bolted  to  the  drum.  A  screw  and  nut  mechanism, 
rfot  shown  in  the  figure,  serves  to  advance  the  electrodes  when 
necessary,  and  electrical  connection  is  made  by  a  stout  copper 
tube  sliding  through  an  insulated  stuffing  box. 

The  charge  is  a  mixture  of  sand  and  coke  or  carborundum,  and 
fills  the  drum  to  at  least  1 2  in.  above  the  electrodes.  It  is  covered 
with  fire-bricks  except  in  the  center  where  a  graphite  ring  having  a 

1  Reproduced  from  Fig.  i  in  Dr.  H.  N.  Potter's  paper,  "The  Production  of 
Monox,"  loc.  cit. 


MONOX  365 

7-in.  opening  is  placed.  Part  of  the  charge  is  dug  away,  so  that  the 
gases  shall^blow  out  through  the  ring. 

Above  the  furnace  is  attached  an  iron  cylinder,  7  ft.  in  diameter 
with  conical  ends,  which  is  connected  to  a  pump  for  maintaining  a 
partial  vacuum.  The  cylinder  is  cooled  externally  by  water  and 
contains  mechanical  scrapers  for  collecting  the  monox  which  con- 
denses on  the  inner  surface. 

Tubular  rubber  gaskets  are  employed  for  making  air-tight  joints 
between  different  parts  of  the  apparatus  and  for  rendering  air- 
tight the  doors  which  afford  access  to  the  terminal  boxes.  Water 
under  pressure  is  supplied  to  the  gaskets,  expanding  them  until  a 
tight  joint  is  obtained,  and  cooling  the  gasket  and  adjacent  parts. 

The  furnace  was  operated  by  an  alternator  specially  designed  for 
supplying  a  constant  amount  of  power  to  the  electric  arc  without 
the  need  of  any  regulating  devices. 

In  spite  of  the  successful  work  that  was  done  in  the  production 
and  utilization  of  monox,  the  author  learns  from  Dr.  Potter  that 
its  manufacture  was  discontinued,  for  financial  reasons,  in  the  year 
1907.  With  regard  to  the  composition  of  monox,  Dr.  Potter  con- 
siders that  silicon  monoxide  dissociates  during  cooling  into  silicon 
and  silica, 


=  Si+SiO2, 

just  as  carbon  monoxide  dissociates  into  carbon  and  carbon  monoxide, 

2CO=C+C02. 

If  quickly  cooled,  as  in  the  production  of  monox,  the  dissociation  only 
proceeds  to  a  small  extent,  and  the  product  consists  largely  of  SiO. 
The  amount  of  SiO  actually  present  cannot  be  determined  by  analy- 
sis, but  there  is  evidence  of  the  presence  of  this  compound  in  the 
product  monox. 


CHAPTER  XIV 

ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES 
ELECTROLYSIS 

When  an  unidirectional  or  continuous  electric  current  flows 
through  a  fused  salt,  or  a  solution  of  a  salt  in  water,  the  salt,  or 
the  water,  is  broken  up  by  the  current  into  two  parts.  One  of 
these  parts  is  hydrogen,  or  a  metal,  which  is  liberated  at  the  cathode 
or  electrode  through  which  the  current  leaves  the  liquid,  while  the 
remainder  of  the  salt,  or  of  the  water,  is  liberated  at  the  anode  or 
electrode  by  which  the  current  enters  the  liquid.  Thus: 

2NaCl  (electrolyzed)  =  2Na  (at  cathode)  +Ch  (at  anode).  That 
is  to  say,  when  fused  common  salt  is  electrolyzed,  sodium  is  set 
free  at  the  cathode  and  chlorine  at  the  anode. 

CuSO4  (in  aqueous  solution)  =  Cu  (at  cathode)  +S04  (at  anode). 
That  is  to  say,  the  electrolysis  of  a  solution  of  copper  sulphate  in 
water  liberates  copper  at  the  cathode  while  SO4  is  set  free  at  the 
anode.  The  final  result  of  the  operation  will  depend,  however, 
upon  the  nature  of  the  anode.  If  this  is  of  platinum,  or  carbon,  and 
is  not  attacked  by  the  SO4,  the  latter  will  react  with  the  water  of  the 
solution  and  will  form  sulphuric  acid  and  oxygen,  thus  : 


and  the  end  products  of  the  electrolysis  will  be  copper  at  the  cathode, 
oxygen  at  the  anode  and  sulphuric  acid  in  the  solution.  If,  how- 
ever, the  anode  were  made  of  copper  or  some  other  metal  that 
would  be  acted  on  by  the  SO4,  this  reaction  would  take  place: 

Cu+SO4  =  CuSO4. 

The  copper  sulphate  solution  would  thus  be  regenerated,  no  oxygen 
would  be  liberated,  and  the  only  result  of  the  operation  would  be  a 
transfer  of  copper  from  the  anode  to  the  cathode. 

The  latter  case  is  exemplified  in  the  electrolytic  refining  of  copper, 
the  anode  consisting  of  impure  copper,  which  constantly  dissolves 
under  the  action  of  the  current,  while  pure  copper  is  deposited  on  the 

366 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES    367 

cathode.  When  it  is  desired  to  extract  a  metal  from  the  fused  salt 
or  solution  in  which  it  is  contained,  the  anode  should,  if  possible,  be 
insoluble  in  whatever  is  set  free  at  its  surface;  or,  if  this  is  impossi- 
ble, it  should  be  inexpensive,  as  it  will  be  dissolved  in  proportion  as 
the  other  metal  is  recovered. 

In  the  equation  given  above  for  the  electrolysis  of  fused  common 
salt,  chlorine  and  sodium  are  the  end  products.  An  aqueous  solu- 
tion could  not  be  used  for  the  production  of  sodium,  as  the 
water  would  react  with  the  sodium,  forming  caustic  soda  and 
hydrogen. 

In  the  electrolysis  of  a  fused  mixture  of  two  salts  or  of  a  solution 
of  a  salt  in  water,  the  current  breaks  up  the  compound  which  is  the 
least  stable;  thus  in  a  solution  of  copper  sulphate  in  water,  the  cur- 
rent separates  the  copper  sulphate  into  its  components,  and  not 
the  water;  but  in  a  solution  of  aluminium  sulphate,  the  water  and 
not  the  aluminium  salt  is  decomposed.  It  is  necessary,  therefore, 
to  employ  a  solvent  that  is  more  stable  than  the  salt  it  is  desired  to 
decompose,  or,  failing  this,  to  use  the  pure  salt  in  a  state  of  fusion. 
This  is  why  the  ex  traction  of  aluminium  from  its  ore  is  carried  out  in  a 
fused  mixture  of  fluorides  instead  of  in  an  aqueous  solution. 

In  the  electrolysis  of  solutions  a  definite  amount  of  electricity 
in  passing  through  the  solution  will  always  produce  a  definite 
amount  of  decomposition.  This  amount  is  always  the  same  for 
the  same  solution,  and  in  different  solutions  chemically  equivalent 
amounts  of  decomposition  are  effected.  A  current  of  i  ampere 
flowing  through  acidulated  water  for  i  second  will  liberate 
0.0104  mg-  of  hydrogen,  and  in  any  other  solution  the  weight  of 
the  metal  liberated  will  be  0.0104  mg->  multiplied  by  the  atomic 
weight  of  the  metal  and  divided  by  the  valency  of  the  metal  in  the 
particular  solution.  Thus,  the  amount  of  the  monovalent  metal 
sodium  that  would  be  set  free  per  second  would  be  0.0104  mg.X23, 
the  atomic  weight  of  sodium,  or  0.239  mg.;  while  the  weight  of 
copper  deposited  would  be  0.0104  rng.X63.2,  the  atomic  weight  of 
copper,  or  0.657  mg.  in  cuprous  salts,  such  as  Cu2Cl2,  in  which 
copper  is  monovalent,  while  in  the  more  usual  cupric  salts,  such  as 
CuSC>4,  in  which  the  metal  is  divalent,  only  half  that  amount 
would  be  deposited  by  the  current.  The  amount  of  metal  actually 
obtained  as  the  result  of  electrolysis  is  frequently  less  than  the 
calculated  weight  on  account  of  secondary  reactions,  such  as  the 
metal  redissolving  in  the  electrolyte,  hydrogen  being  liberated  instead 
of  the  metal,  leakage  of  the  current,  etc.,  and  the  ratio  of  the  metal 


368  THE  ELECTRIC  FURNA  CE 

actually  deposited  to  the  theoretical  quantity  is  known  as  the 
current  efficiency,  as  it  shows  what  proportion  of  the  current  is 
effective  in  liberating  the  metal. 

The  electrical  energy  required  to  produce  a  definite  weight 
of  a  metal,  by  the  electrolysis  of  a  chemical  compound  of  the  metal, 
depends  not  only  on  the  number  of  ampere  hours  needed  to  liberate 
the  weight  of  metal,  but  also  on  the  voltage  maintained  during  the 
operation;  that  is  on  the  electrical  pressure  needed  to  drive  the 
electric  current  through  the  electrolyte  so  as  to  produce  the  de- 
composition. Each  solution  has  a  definite  electrical  pressure 
which  must  be  exceeded  before  electrolysis  will  take  place,  and 
the  working  voltage  must  be  decidedly  above  the  minimum  in 
order  to  drive  a  rapid  current  of  electricity  through  the  solution. 
The  passage  of  the  current  also  produces  heat,  the  amount  being 
proportional  to  the  square  of  the  current  and  to  the  resistance  of 
the  electrolytic  cell,  while,  as  has  been  noted,  the  amount  of  metal 
deposited  is  proportional  to  the  current  alone.  As  the  chemical 
work  performed  is  proportional  to  the  current,  and  the  heat  gener- 
ated to  the  square  of  the  current,  the  efficiency  will  be  greater  as 
the  current  is  smaller.  In  electrolysis  at  furnace  temperatures, 
however,  it  is  often  convenient  to  heat  the  electrolyte  electrically 
instead  of  by  the  external  application  of  fuel  heat,  and  in  such 
cases  the  production  of  heat  by  the  current  cannot  be  regarded  as 
waste. 

The  nature  of  the  anode  has  a  great  effect  upon  the  voltage 
needed  for  electrolysis.  Thus,  in  electrolyzing  a  solution  of  copper 
sulphate  with  an  anode  that  does  not  dissolve,  copper  and  oxygen 
will  be  liberated,  the  electric  current  will  have  to  do  the  work  of 
separating  these  elements,  and  a  pressure  of  more  than  one  volt  will 
be  needed;  but  if  the  anode  is  made  of  copper,  the  metal  will  be  re- 
moved from  the  anode  as  fast  as  it  is  deposited  on  the  cathode, 
no  chemical  work  will  be  done,  and  the  smallest  voltage  will 
suffice  to  produce  electrolysis. 

It  is  possible  to  calculate,  from  available  data,  the  amount  of 
energy  needed  to  separate  a  definite  weight  of  a  compound  into  its 
elements,  and  the  relation  of  this  to  the  electrical  energy  actually 
required  to  produce  this  decomposition  is  the  energy  efficiency  of 
the  process. 

When  an  anhydrous  salt  or  mixture  of  salts  is  used  as  an  electrolyte 
it  must  be  heated,  usually  to  a  red  heat,  to  render  it  fluid,  and  its 
electrolysis  may  be  classed  as  a  furnace  operation. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES     369 


TABLE  XXIV.— ELECTROCHEMICAL  EQUIVALENTS 


Element 

Atomic 
weight 
(0  =  i6) 

Nature  of 
electrolyte 

Symbol 
and 
valency 

Amount  deposited 
by  i  ampere 

Mg.  per 
sec. 

Lbs.  per 
hour 

Aluminium  
Antimony  
Arsenic  

27.1 

120.  2O 
74.96 

137-37 
208.0 
79.92 
1  1  2  .  40 
40.C9 
35.46 
52.Q 
58.97 
63-57 
63-57 
I9.O 
197.20 
1.008 
126.92 
55-85 
55-85 

207.  10 

7.00 
24.32 

54-93 
200.  o 
200.  o 
58.68 
16.00 
iQ5-o 
39.10 
107.88 
23.00 
87.62 
32.07 
119.0 
119.0 
6s.  37 

Oxides  and  salts.  .  . 
Sulpho-salts  
Arsenites  
Haloid  salts  
Bismuth  salts  
Bromides  
Salts  

Al.    Ill 
Sb.    Ill 
As.    Ill 
Ba      II 
Bi.    Ill 
Br.       I 
Cd.     II 
Ca.     II 
Cl.        I 
Cr.    Ill 
Co.     II 
Cu.       I 
Cu.     II 
F.         I 
Au.   Ill 
H.        I 
I.          I 
Fe.      II 
Fe     III 
Pb.     II 
Li.        I 
Mg.    II 
Mn.    II 
Hg.       I 
Hg.     II 
Ni.     II 
0.       II 
Pt.     IV 
K.        I 
Ag.       I 
Na.       I 
Sr.      II 
S.        II 
Sn.      II 
Sn.     IV 
Zn.     II 

0.0936 
0.4152 
0.2589 
o.  7118 
0.7185 
0.8282 
0.5824 
0.2077 

0-3675 
0.1796 

0-3055 
0.6587 
0.3294 
o.  1969 
0.6812 
0.0104 
i-3i5o 
o.  2894 
o.  1929 

1.0730 
0.0725 

o.  1260 

0.2846 
2.0725 
1.0363 

0.3040 
0.0829 

0.5052 
0.4052 
1.1179 
0.2383 
0.4540 

o.  1662 
0.6166 
0.3083 
o.  3387 

0.000743 
0.003295 
0.002055 
o  .  005649 
0.005703 
0.006573 
0.004622 
0.001648 
0.002917 
0.001425 
0.002425 
0.005228 
0.002614 
0.001563 
0.005406 
0.000082 
0.010437 
0.002297 
0.001531 
0.008516 
0.000575 

O.OOIOOO 

0.002259 
0.016450 
0.008224 
0.002413 
0.000658 
0.004010 
0.003216 
0.008873 
0.001891 

0.003^03 

0.001319 
0.004894 
0.002447 
0.002688 

Barium       

Bismuth  
Bromine  

Cadmium         .  . 

Calcium  

Haloid  salts  
Chlorides  

Chlorine  
Chromium 

Chromic  salts  
Cobaltous  salts.  .  .  . 
Cuprous  salts  
Cupric  salt  .  . 

Cobalt  

Copper  .  . 

Copper  
Fluorine  

Fluorides  
Haloid  salts  
Aqueous  solutions. 
Iodides  

Gold  

Hydrogen  
Iodine  
Iron 

Ferrous  salts  
Ferric  salts  

Iron  

Lead  
Lithium 

Salts  

Haloid  salts  
Haloid  salts  

Magnesium  
Manganese  
Mercury  
Mercury  
Nickel  

Manganous  salts.  .  . 
Mercurous  salts.  .  . 
Mercuric  salts  
Nickelous  salts  
Oxides 

Oxygen  
Platinum  
Potassium  .  . 

Haloid  salts  
Salts  

Silver 

Salts 

Sodium  
Strontium 

Salts  
Haloid  salts. 

Sulphur  

Sulphides  

Tin  
Tin.  

Stannous  salts  
Stannic  salts  
Salts.. 

Zinc.  .  . 

24 


370  THE  ELECTRIC  FURNACE 

ELECTROLYTIC  EXTRACTION  PROCESSES 

Electrolytic  processes  may  be  divided  into  two  classes  according 
as  they  are  intended  for  the  recovery  of  a  metal  or  other  element 
from  its  compound  or  for  purifying  a  metal  which  has  already  been 
obtained. 

Many  examples  of  both  classes  might  be  quoted,  where  the  elec- 
trolysis is  carried  out  in  aqueous  solutions,  but  the  electrolysis  of 
anhydrous  electrolytes  is  almost  entirely  directed  to  the  separation 
of  a  chemical  compound  into  its  constituents,  as  in  the  extraction 
of  aluminium  from  alumina,  or  chlorine  and  sodium  from  common 
salt. 

The  Acker  Process  for  Caustic  Soda  and  Chlorine. — In  this 
process  fused  common  salt  or  sodium  chloride  is  electrolyzed, 
using  carbon  anodes  by  which  the  current  enters  the  liquid,  and 
molten  lead  for  the  cathode  by  which  the  current  leaves.  The 
salt  is  broken  up  into  chlorine,  which  is  liberated  at  the  anode 
and  is  led  away  and  used  for  making  bleaching  powder,  and  sodium, 
which  is  liberated  at  the  cathode  and  forms  an  alloy  with  the  lead. 
The  lead  containing  the  sodium  is  then  treated  with  steam,  which 
combines  with  the  sodium  to  form  caustic  soda. 

The  following  reactions  take  place: 

2NaCl  (electrolyzed)  =  C12  (liberated  at  the  anode)  +  sNa  (alloying 
with  the  lead  cathode). 

2Na  (alloyed  with  the  lead)+2H2O  (the  jet  of  steam)  =  2NaOH+ 
H2. 

The  ingenious  arrangement  by  which  this  is  accomplished  is 
illustrated  in  Fig.  147.  The  apparatus  consists  of  an  irregular 
shaped  cast-iron  vessel,  about  5  ft.  long,  which  is  divided  into 
three  compartments,  A,  B,  and  C,  with  the  connecting  channel, 
R.  The  larger  compartment,  A,  contains  melted  salt,  S,  resting 
on  a  thin  layer  of  molten  lead  which  is  caused  to  circulate  as  shown 
by  the  arrows.  Four  electrodes,  E,  of  graphitized  carbon  are 
immersed  in  the  fused  salt  and  form  the  anode  of  the  electrolytic 
celt,  being  connected  to  the  positive  cable  from  a  dynamo.  The 
iron  tank  is  connected  at  the  point  H  to  the  negative  cable,  thus 
making  the  molten  lead,  the  cathode.  The  electric  current  passes 
from  the  carbon  electrodes  through  the  melted  salt  to  the  fused 
lead  on  which  the  salt  rests.  In  passing  through  the  salt,  chlorine 
is  liberated  at  the  carbon  electrodes  and  escapes,  being  drawn  away 
by  a  fan,  while  the  sodium  is  liberated  at  the  surface  of  the  lead  and 
alloys  with  it.  In  the  small  compartment,  B,  a  jet  of  steam  in- 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES    371 

troduced  by  the  pipe,  F,  serves  to  blow  the  lead  up  the  vertical 
pipe,  P,  and  over  into  the  third  compartment,  C,  from  which  the 
lead  returns  by  the  passage,  R,  to  the  first  compartment.  The 
lead  entering  B  is  charged  with  sodium,  and  when  it  meets  the 
steam  in  the  pipe,  P,  the  sodium  combines  with  the  steam,  forming 
anhydrous  caustic  soda,  which  floats  on  the  lead  in  C  and  overflows 
by  the  spout,  D,  and  hydrogen,  which  escapes  at  D  and  burns. 
The  compartment  A  is  lined,  above  the  level  of  the  lead,  with  mag- 
nesite-bricks,  M,  and  the  cover  is  formed  of  fire-brick  tiles,  T.  The 
salt  to  be  used  in  the  process  is  warmed  on  the  top  of  the  furnace  and 
then  introduced  through  charging  holes  in  the  roof.  Each  anode 
consists  of  a  block  of  graphitized  carbon  14  in.  long,  7.5  in.  wide, 
and  3  in.  thick,  which  is  supported  by  two  5-in.  carbon  rods  passing 
through  the  top  of  the  furnace.  The  carbon  blocks  are  lowered 


JULOLDt 


FIG.  147. — Acker  furnace  for  caustic  soda. 

until  within  3/4  in.  of  the  molten  lead.  The  current  used  is  8,200 
amperes,  the  voltage  of  each  furnace  or  pot  being  only  6  or  7. 
From  40  to  45  pots  are  used  at  once,  being  connected  in  series,  so 
that  the  same  current  passes  through  them  all,  and  the  total  voltage 
necessary  is  275;  about  3,000  h.p.  being  supplied  at  the  generating 
station,  which  is  1,500  ft.  away.  The  current  density  at  the  anodes 
is  about  2,750  amperes  per  square  foot,  and  this  is  sufficient  to  keep 
the  salt  at  a  temperature  of  850°  C.,  which  is  a  bright  red  heat,  and 
75°  C.  above  the  melting-point  of  the  salt,  while  it  is  far  above 
the  melting-point  of  lead.  The  cast-iron  vessel  which  forms  the 
furnace  is  set  in  brickwork  which  reduces  the  loss  of  heat  by 
radiation  and  conduction.  The  output  of  each  furnace  is  25  Ib. 
of  caustic  per  hour,  which  is  93  per  cent,  of  the  amount  which 
should  theoretically  be  produced  by  the  current,  but  the  voltage 
is  considerably  higher  than  is  required  by  theory,  as  nearly  half  of 


372  THE  ELECTRIC  FURNACE 

the  energy  of  the  current  is  needed  to  keep  the  furnace  at  the  high 
temperature  of  fused  salt.  The  caustic  soda  in  C  is  fused  and 
practically  anhydrous,  so  that  it  is  ready  for  market  without  any 
boiling  down,  such  as  is  required  when  aqueous  solutions  are  used 
for  electrolysis.  The  Acker  process  has  been  described  by  C.  E. 
Acker  in  the  Transactions  of  the  American  Electrochemical  Society, 
vol.  i,  1902,  and  by  Prof.  Richards,  Electrochemical  Industry, 
vol.  i,  1902,  p.  54. 

The  Castner  Sodium  Process. — This  is  the  standard  method  of 
making  that  metal,  thousands  of  tons  per  annum  being  made  in 
this  manner.  In  this  process,  fused  anhydrous  caustic  soda  is 
electrolyzed,  using  nickel  for  the  anode,  and  carbon,  or  some  metal 
such  as  iron,  for  the  cathode.  The  products  of  the  operation  are 
sodium  and  hydrogen  at  the  cathode,  and  oxygen  at  the  anode,  all 
in  equal  atomic  proportions. 

The  following  reactions  probably  take  place: — 
,    2NaOH    (electrolyzed)  =  2Na    (at    cathode) +2 HO    (at   anode). 

4.HO  (at  anode)  =  2H2O+O2. 

2H2O  (electrolyzed)  =  2H2  (at  cathode) +O2  (at  anode). 

Or,  put  into  one  equation : — 

2NaOH  (electrolyzed)  =  [Na2+H2]  (at  cathode) +O2  (at  anode). 

As  the  sodium  is  lighter  than  tjie  fused  caustic,  it  floats  to  the 
top,  and  great  difficulty  is  experienced  in  preventing  it  from  burn- 
ing in  the  air  or  in  the  oxygen  liberated  at  the  anode,  which  also 
rises  to  the  surface.  In  the  Castner  apparatus,  Fig.  148,  this  is 
accomplished  by  the  metal  cylinder  E,  from  the  lower  edge  of 
which  a  cylinder  of  nickel  gauze  is  continued  down  between  the 
nickel  anode  C  and  the  cathode  D.  The  sodium  rises  within  this 
cylinder  and  collects  at  F,  from  which  it  may  be  ladled,  or  may 
overflow  through  a  spout,  while  the  oxygen  rises  outside  the  gauze 
cylinder,  and  is,  therefore,  unable  to  attack  the  sodium.  The 
hydrogen  rises  with  the  sodium  inside  the  cylinder  and  escapes 
through  the  holes  in  the  cover.  The  use  of  the  gauze  cylinder 
allows  the  anode  and  cathode  to  be  brought  very  close  to  each 
other,  being  only  i  in.  apart,  without  danger  of  the  sodium 
meeting  the  oxygen,  and  in  this  way  the  resistance  of  the  apparatus 
is  kept  low,  and  a  high  electrical  efficiency  can  be  obtained. 

The  apparatus  consists  of  a  cast-iron  pot  A,  set  in  brick  work, 
B,  and  heated  if  necessary  by  a  ring  of  gas  burners,  H,  to  a  tem- 
perature very  little  above  the  melting-point  of  the  caustic  soda. 
The  cathode,  D,  is  supported  in  position  and  insulated  from  the 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES     373 

iron  pot  by  means  of  the  tube,  G,  which,  being  closed  at  the  bottom 
by  a  ring  of  insulating  material,  such  as  porcelain,  is  filled  with  the 
fused  caustic  soda,  which  is  then  allowed  to  solidify.  The  tube  is 
kept  a  little  cooler  than  the  rest  of  the  apparatus,  and  the  caustic 
in  G,  therefore,  remains  solid,  and  supports  and  insulates  the  cathode. 
It  is  very  important  that  the  fused  caustic  should  not  be  heated 
far  above  its  melting-point,  because  the  sodium  would  then  rapidly 
redissolve  in  it.  The  caustic  melts  at  about  300°  C.,  and  should 
be  kept  not  more  than  10°  above  this;  90  per  cent,  of  the  theoretical 


^^ 
FIG.  148. — Castner  sodium  furnace. 

quantity  of  sodium  being  then  obtained.  If  heated  20°  above  its 
melting-point  no  sodium  would  be  produced,  as  it  would  dissolve 
as  rapidly  as  it  formed.  A  pot  18  in.  in  diameter  and  2  ft.  deep 
will  hold  250  Ib.  of  melted  caustic,  and  takes  a  current  of  1,200 
amperes  at  five  volts.  The  current  density  is  2,000  amperes  per 
square  foot  at  the  cathode  and  1,500  at  the  anode.  At  the  Niagara 
Electrochemical  Company's  plant  in  1902 1  there  were  120  such 
pots,  employing,  in  all,  1,000  h.p.,  and  producing  "at  an  assumed 
efficiency  of  90  per  cent.,"  about  25  Ib.  each  per  day,  or  3,000  Ib. 

1  Richards,  Electrochem.  Ind.,  vol.  i,  1902,  p.  15. 


374  THE  ELECTRIC  FURNACE 

for  the  whole  plant.  The  consumption  of  power  was  about  8  h.p.- 
hours  per  pound  of  sodium. 

Of  the  total  annual  production  of  sodium,  about  1,50x5  tons  are 
used  for  cyanide  making,  1,500  tons  for  making  sodium  peroxide, 
and  500  tons  are  sold  in  the  metallic  state, 

The  Ashcroft  Sodium  Process.1 — This  is  an  attempt  to  produce 
sodium  from  common  salt  instead  of  from  the  more  expensive 
caustic  soda.  Common  salt  has  so  high  a  fusing  temperature  that 
it  cannot  be  electrolyzed  directly  for  the  metal  sodium,  as  this 
would  be  volatilized.  In  the  Acker  process  sodium  is  produced, 
but  only  as  an  alloy  with  molten  lead,  from  which  it  is  recovered 
as  caustic  soda.  The  Ashcroft  process,  illustrated  in  Fig.  149, 
consists  in  electrolyzing  fused  salt  in  a  tank,  A,  using  lead  as  the 
cathode  to  retain  the  resulting  sodium,  and  then  carrying  the  sodium 
lead  alloy  to  a  second  tank,  B,  where  it  becomes  the  anode,  in  a  bath 
of  fused  caustic  soda  kept  just  above  its  melting-point.  In  this 
tank  metallic  sodium  is  liberated  at  the  cathode,  C,  and  floating 
upward  is  caught  within  the  hood  £>,  and  overflows  through  the 
pipe  E. 

The  reactions  that  take  place  can  be  made  clear  by  the  following 
equations: 

In  A— 2NaCl  (electrolyzed)  =  C12  (liberated  at  the  anode) +2Na 
(dissolving  in  the  lead  cathode)  (i) 

In  B— NaOH  (electrolyzed)=HO  (at  anode)  +Na  (liberated  at 

the  cathode) (2) 

Na  (in  lead  alloy) +HO  =  NaOH (3) 

In  A,  with  an  insoluble  anode,  the  salt,  which  forms  the  elec- 
trolyte, is  broken  up  into  chlorine  and  sodium.  In  B,  the  caustic 
soda  electrolyte  is  re-formed  by  reaction  (3)  as  fast  as  it  is  destroyed 
by  reaction  (2). 

The  products  of  the  first  tank  are  chlorine,  which  is  piped  away 
and  utilized,  and  sodium  as  an  alloy  with  lead;  while  common  salt 
is  consumed.  The  second  tank  yields  sodium  only,  which  it  takes 
from  the  lead  alloy.  The  fused  caustic,  which  serves  as  electrolyte, 
is  not  destroyed,  but  merely  serves  as  a  carrier  for  the  sodium. 
The  electric  current  will  liberate  twice  as  much  sodium  as  in  the 
Castner  process,  because  only  sodium  is  set  free  at  the  cathode, 

1  Ashcroft,  Trans.  Am.  Electrochem.  Soc.,  vol.  ix,  1906,  p.  123;  Electrochem. 
and  Met.  Ind.,  vol.  iv,  1906,  p.  218. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES    375 

while   in    the  older   process   equal   equivalents    of    sodium    and 
hydrogen  were  set  free. 

As  the  tank  A  must  be  hot  enough  to  fuse  salt,  that  is  nearly 
800°  C.,  while  the  tank  B  is  little  more  than  300°  C.,  the  lead  sodium 
alloy  must  be  cooled  during  its  passage  from  A  to  B,  and  the  lead 
returning  from  B  to  A  must  be  reheated.  This  is  accomplished  by 
a  twin  pipe  P  of  considerable  length  connecting  the  two  vessels, 
so  that  the  alloy  flowing  from  A  to  B  gives  up  its  heat  to  the  lead 
flowing  from  B  to  A.  The  pipe  is  folded  on  itself  for  compactness, 
only  a  part  being  shown  in  the  drawing.  The  method  of  producing 


FIG.  149. — Ashcroft  sodium  furnace. 

a  continuous  circulation  of  the  lead  is  also  very  ingenious,  and  con- 
sists in  producing  electro-magnetically,  a  rotation  of  the  lead  alloy 
in  A,  and  in  providing  a  suitable  baffle  F,  and  openings  G  and  H 
in  the  end  of  the  twin  pipe  which  enters  A,  so  that  the  rotating 
alloy  is  forced  to  pass  from  A  to  B,  circulate  in  B,  and  after  giving 
up  its  sodium  return  to  A .  The  openings  at  both  ends  of  the  twin 
pipe  are  so  arranged  that  the  richest  of  the  alloy  in  A  is  skimmed 
off  and  is  conducted  to  B,  where  it  passes  to  the  surface,  and  so 
gives  up  its  sodium  before  returning  to  A.  The  rotation  of  the 
metal  is  shown  by  arrows  in  the  figure.  The  rotation  in  A  is  caused 
by  a  coil  of  wire  W  within  the  cast-iron  tank,  but  separated  from 


376  THE  ELECTRIC  FURNACE 

the  molten  salt  by  the  lining  of  magnesite  or  similar  material. 
The  whole  current  used  in  the  process  passes  through  this  coil 
and  produces  a  strong  magnetic  field  within  A,  the  lines  of  mag- 
netic force  pointing  upward  as  shown  by  the  arrows.  After  leav- 
ing the  coil  the  current  passes  to  the  carbon  anode  N,  and  then 
through  the  fused  salt  to  the  molten  lead.  The  anode  being  small 
and  central,  and  the  lead  cathode  being  more  extended,  the  direc- 
tion of  the  current  though  mainly  vertical,  will  be  partly  horizontal, 
and  so  will  cut  the  lines  of  magnetic  force.  The  result  will  be  hori- 
zontal rotation  of  the  molten  contents  of  A ,  and,  as  has  been  stated, 
this  leads  to  the  desired  circulation  of  lead  from  A  to  B  and  back 
again.  The  tanks  A  and  B  are  made  of  cast-iron,  and  heated  ex- 
ternally by  fuel  as  well  as  internally  by  the  passage  of  the  current. 
A  is  provided  with  two  openings,  /  and  K,  one  of  which  has  a  hopper 
for  charging  in  the  salt,  while  the  other  serves  to  remove  the  chlorine. 
In  B,  the  cathode  C  is  globular  in  form,  allowing  the  sodium  which 
deposits  around  it  to  pass  easily  upward  into  the  hood  D.  The 
cathode  is  insulated  from  the  bottom  by  a  layer  of  solidified  caustic 
as  in  the  Castner  apparatus,  and  is  hollow,  thus  allowing  of  cooling 
by  air  or  other  fluids  if  the  temperature  becomes  too  high.  The 
hood  D  is  connected  to  the  iron  cover  of  B,  and  thus  with  the  sodium- 
lead  anode,  so  that  there  is  no  tendency  for  sodium  to  form  on  any 
part  of  the  tank  except  the  cathode  C.  The  method  for  producing 
a  circulation  of  the  lead  does  not  sound  very  efficient,  but  it  is 
stated  to  work  well  in  a  furnace  using  some  2,000  or  3,000  amperes, 
and  the  whole  operation  is  reported  to  be  working  satisfactorily. 
The  voltage  needed  will  be  about  7  volts  in  A ,  that  is,  the  same  as 
in  the  Acker  process,  and  about  2  volts  in  B,  or,  in  all,  9  volts. 
The  process  should  show  marked  economies  in  comparison  with 
the  Castner  method. 

Carrier  Sodium  Process. — Another  attempt  to  obtain  metallic 
sodium  by  the  electrolysis  of  fused  common  salt  has  been  made  by 
C.  F.  Carrier,1  who  has  designed  a  furnace  which  is  similar  in  princi- 
ple to  the  Ashcroft  furnace,  but  somewhat  different  in  design. 

The  furnace,  Fig.  150,  consists  of  a  cast-iron  pan  set  in  masonry 
and  divided  into  two  compartments,  an  anode  compartment,  A, 
and  a  cathode  compartment,  C.  The  pan  contains  molten  lead, 
L,  which  serves  as  a  carrier  for  the  sodium  between  the  anode  and 
cathode  compartments.  The  lead  is  caused  to  circulate  through 

1C.  F.  Carrier,  Jr.,  "Metallic  Sodium  from  Fused  Sodium  Chloride,"  Met. 
and  Chem.  Eng.,  viii,  1910,  p.  253. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES     377 

a  passage  P  by  means  of  a  screw  propeller.  The  anode  compart- 
ment, A,  contains  fused  sodium  chloride.  The  anode  is  made  of 
graphite  and  is  protected  by  an  earthenware  sleeve.  The  electric 
current,  passing  through  this  compartment,  electrolyzes  the  sodium 
chloride,  liberating  sodium  which  alloys  with  the  molten  lead,  and 
chlorine  which  escapes  through  the  openings  F.  The  lead  contain- 
ing the  sodium  passes  by  the  action  of  the  propeller  through  the 
cathode  compartment,  C,  where  it  serves  as  anode.  The  electric 
current,  passing  from  the  lead  in  this  compartment  to  the  iron  cathode 
B,  causes  the  sodium  in  the  molten  lead  to  dissolve  in  the  electrolyte 


FIG.  150. — Carrier  sodium  furnace. 

and  to  redeposit  on  the  cathode,  from  which  it  passes  in  a  molten 
condition  and  overflows  by  a  spout  S.  The  cathode  is  insulated 
from  the  iron  Casing  of  this  compartment.  The  lining  E  and  cover 
D  of  the  anode  compartment  are  made  of  fire-brick.  The  spaces 
in  the  masonry  under  the  pan  contain  gas  burners  which  help  to 
keep  the  electrolyte  in  a  state  of  fusion. 

In  the  anode  compartment  the  electrolyte  consists  at  first  of  a 
mixture  of  three  molecules  of  sodium  chloride,  three  molecules  of 
potassium  chloride  and  two  molecules  of  calcium  chloride;  a  mix- 
ture which  is  far  more  fusible  than  sodium  chloride  and  yields 
almost  pure  sodium  at  the  cathode.  Sodium  chloride  alone  is 
added  after  the  start.  The  electric  current  passing  through  this 
compartment  liberates  chlorine  at  the  anode  and  sodium  at  the 


378 


THE  ELECTRIC  FURNACE 


cathode  where  it  alloys  with  the  molten  lead.  In  the  cathode 
compartment  fused  caustic  soda  was  at  first  employed,  as  in  Ash- 
croft's  apparatus,  but  this  was  found  to  yield  no  sodium.  Ap- 
parently the  sodium  became  oxidized  and  dissolved  in  the  fused 
caustic  which  was  at  a  temperature  between  700°  C.  and  900°  C. 
and  not  entirely  protected  from  the  air.  It  will  be  remembered 
that  in  the  Ashcroft  apparatus,  in  which  sodium  is  obtained  in  a 
similar  manner,  the  caustic  soda  is  kept  at  little  more  than  300° 
C.  An  electrolyte  of  sodium  and  potassium  chlorides  was  then 
tried  instead  of  the  caustic  soda  and  was  found  to  yield  sodium 
satisfactorily. 


y/7////////////^^^ 

FIG.  151. — Virginia  Company's  sodium  furnace. 

In  the  anode  compartment  the  graphite  anode  had  an  effective  area 
of  43  sq.  in.,  the  molten  lead  cathode  had  an  area  of  242  sq.  in.,  and  a 
current  of  600  to  700  amperes  was  used.  The  current  in  the  cathode 
compartment  was  reduced  to  about  75  per  cent,  of  that  in  the  anode 
compartment  (to  allow  for  inefficiency  in  that  compartment)  so  that, 
as  more  sodium  would  thus  be  supplied  to  the  lead  in  the  anode  com- 
partment than  would  leave  it  in  the  cathode  compartment,  the  lead 
would  never  become  entirely  free  from  sodium.  Six  to  8  volts  were 
needed  for  the  furnace,  but  the  experiments  were  not  continued 
long  enough  to  determine  the  yield  of  sodium,  which  was  liable  to  be- 
come oxidized  during  its  removal  from  the  furnace.  In  starting 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES    379 

the  operation  the  gas  burners  were  not  sufficient  to  melt  the  mixed 
chlorides  and  this  was  done  by  means  of  carbon  resistor-rods  which 
were  driven  into  the  face  of  the  anode  and  carried  the  current  from 
that  to  the  molten  lead  cathode.  These  rods  became  very  hot  and 
melted  the  salt  which  was  piled  around  them;  the  rods  were  then 
broken  off  and  electrolysis  took  place  through  the  fused  salt. 

Virginia  Electrolytic  Company's  Sodium  Furnace.1 — In  the  year 
1910  the  Niagara  Electrochemical  Company,  using  the  Castner  proc- 
ess, and  the  Virginia  Electrolytic  Company  were  the  only  producers 
of  sodium  in  the  United  States.  The  latter  company  produce  sodium 
by  the  electrolysis  of  fused  sodium  chloride  in  the  furnace  shpwn  in 
Fig.  151.  The  furnace  is  circular,  containing  an  annular  graphite 
anode,  AA,  and  hollow  iron  cathode  C.  Electrolysis  of  the  fused 
salt  yields  sodium,  which  rises  within  the  water-cooled  curtain,  BB, 
and  chlorine  which  collects  outside  this  curtain  and  passes  out  by  the 
pipe  D.  The  sodium  flows  down  through  C  and  collects  in  E,  from 
which  it  is  tapped  at  intervals.  The  largest  furnace  constructed 
uses  a  current  of  10,000  amperes. 

Potassium. — The  alkali  metal  potassium  strongly  resembles  sod- 
ium, and  is  obtained  by  electrolysis  of  its  fused  salts  in  substantially 
the  same  manner. 

Magnesium. — This  metal  is  well  known  on  account  of  its  prop- 
erty of  burning  with  a  very  bright  flame,  which  is  made  use  of  for 
photography.  It  has  a  specific  gravity  of  1.74,  melts  at  750°  C.  and 
can  be  rolled  into  sheets  or  ribbons.  It  has  a  very  strong  affinity  for 
oxygen,  burning  when  ignited  and  gradually  corroding  away  if  ex- 
posed to  the  air  at  the  ordinary  temperature.  The  oxide  MgO  and 
carbonate  MgCOs  are  well  known  and  abundant;  the  oxide  being 
of  great  value  as  a  refractory  material  for  lining  electric  furnaces. 

The  metal  is  obtained  by  electrolysis  of  the  fused  chloride,  hav- 
ing been  first  isolated  in  this  manner  by  Bunsen  about  the  year  1852. 
This  process  has  been  carried  out  recently  in  the  laboratory  by  Prof. 
S.  A.  Tucker,2  using  a  fused  mixture  of  magnesium  chloride  and 
potassium  chloride  in  a  graphite  crucible. 
The  charge  consisted  of: 

203  parts  of  crystallized  magnesium  chloride,  MgCl2,6H2O 
74  parts  of  potassium  chloride,  KC1, 

50  parts  of  sublimed  ammonium  chloride,  NH4C1. 
/ 

1  Mineral  Industry,  vol.  xix,  1910,  p.  614. 

2  Prof.  S.  A.  Tucker  and  Mr.  F.  A.  Jouard,  "The  Electrolytic  Preparation  of 
Magnesium,"  Trans.  Am.  Electrochem.  Soc.,  vol.  xvii,  1910,  p.  249. 


380 


THE  ELECTRIC  FURNACE 


That  is,  a  mixture  of  these  chlorides  in  molecular  proportions.  The 
ammonium  chloride  is  added  to  prevent  the  decomposition  of  the 
magnesium  chloride  (with  loss  of  hydrochloric  acid),  during  the  pre- 
liminary fusion. 

The  apparatus  consists  of  a  graphite  crucible,  Fig.  152,  2.75  in. 
in  diameter  and  3.5  in.  high,  surrounded  by  magnesia,  M,  to 
retain  the  heat  as  far  as  possible.  The  crucible  stands  on  a  block 
of  graphite,  B,  into  which  is  threaded  a  rod  of  copper.  The  crucible 
is  made  the  cathode,  and  a  graphite  electrode,  A,  1.25  in.  in  di- 
ameter, forms  the  anode.  The  operation  is  started  by  fusing  the 
mixed  salts  in  a  platinum  dish,  during  which  operation  the  water  of 


FIG.  152. — Tucker  magnesium  furnace. 

crystallization  and  the  ammonium  chloride  are  eliminated,  and 
pouring  the  melted  chlorides  into  the  graphite  crucible,  which  should 
be  hot,  to  prevent  the  salts  chilling  and  stopping  the  operation. 
Chlorine  is  given  off  at  the  anode  and  magnesium  forms  at  the 
cathode.  At  a  low  temperature,  about  450°  C.,  the  metal  forms 
in  a  sponge  and  can  be  removed  and  melted  together  under  a  flux  of 
calcium  fluoride  and  the  electrolyte.  At  a  higher  temperature, 
about  700°  C.,  the  metal  melts  and  floats  to  the  surface  where  it 
probably  unites  with  the  liberated  chlorine.  It  is  therefore  prefer- 
able to  maintain  a  low  temperature.  A  current  of  150  amperes  at 
30  volts  is  suitable  for  this  apparatus. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES     381 

The  operation  can  also  be  carried  out  in  a  Muthmann  crucible,1 
Fig.  153,  which  consists  of  a  copper  tube,  1.25  in.  in  diameter  and 
4  in.  long,  spun  to  a  cone  for  part  of  its  length,  BD,  and  provided 
with  a  water-jacket.  The  cathode,  C,  is  a  rod  of  carbon  or  graphite, 
about  3/4  in.  in  diameter,  wrapped  in  asbestos  paper  between  D 
and  E,  which  makes  a  tight,  non-conducting  joint  between  it  and 
the  copper  tube.  The  anode,  A,  is  supported  above  in  an  adjustable 
holder.  In  using  this  crucible  for  the  electrolysis  of  a  fused  salt,  the 
salt  solidifies  around  the  sides  and  bottom  of  the  crucible,  thus 


FIG.  153. — Muthmann  crucible. 

producing  a  lining  of  the  same  material  as  the  molten  contents. 
The  latter  being  kept  molten  by  the  electric  current  used  for 
electrolysis. 

Calcium. — This  metal,  which  until  recently  was  quite  rare  in  the 
metallic  form,  although  lime,  the  oxide  of  calcium,  is  so  common,  is 
obtained  by  the  electrolysis  of  its  fused  chloride.  It  has  also  been 
produced  by  heating  calcium  carbide  to  a  very  high  temperature 
in  the  electric  furnace.  The  carbide  dissociates,  yielding  calcium 
vapor  which  can  be  condensed.  Calcium  is  less  violent  in  its 

1  Muthmann  Crucible,  S.  A.  Tucker,  Trans.  Am.  Electrcchem.  Soc.,  xvii,  1910, 
p.  256. 


382  THE  ELECTRIC  FURNACE 

reactions  than  sodium  and  will  form  a  valuable  reducing  reagent 
in  certain  metallurgical  operations.  When  calcium  is  added  to 
molten  steel,  it  is  said  to  remove  very  completely  not  only  the 
oxygen,  but  also  any  nitrogen  that  may  be  contained  in  the  steel, 
forming  a  nitride  of  calcium.  Another  compound,  calcium  hydride, 
is  formed  by  heating  calcium  in  an  atmosphere  of  hydrogen.  It  is 
called  "hydrolith"  (hydrogen  stone),  because,  when  placed  in 
water,  it  liberates  a  large  quantity  of  hydrogen.  The  hydrogen  is 
supplied  in  part  by  the  calcium,  which  reacts  with  water,  forming 
lime  and  hydrogen,  and  in  part  from  the  hydrogen  contained  in  the 
calcium  hydride,  as  shown  in  the  equation: 

CaH2+ 2H2O  =  CaH2O2+ 2H2. 

% 
One  kilogram  of  the  hydrolith  is  found  to  yield  as  much  as  one 

cubic  meter  of  hydrogen,  so  that  it  should  be  very  valuable  for 
inflating  balloons  and  for  other  purposes. 

Barium  and  Strontium,  like  calcium,  are  obtained  by  electrolysis 
of  the  fused  chloride. 

Zinc. — The  electrolysis  of  fused  zinc  chloride  is  easily  carried  out 
using  a  carbon  anode  and  a  zinc  cathode.  According  to  the  tempera- 
ture of  the  apparatus,  the  cathode  and  deposited  metal  may  be  solid,1 
or  they  may  be  molten  as  in  the  furnace  shown  in  Fig.  22.  In  the 
former  case  a  current  of  100  or  200  amperes  per  square  foot  of 
cathode  needed  an  E.M.F.  of  3  or  4  volts;  chlorine,  which  is  the  other 
product  of  the  electrolysis,  can  be  piped  away  and  utilized. 

The  electrolysis  of  zinc  chloride  forms  a  part  of  several  processes 
for  the  recovery  of  zinc  from  its  ores,  and  one  of  these  may  now  be 
described. 

The  Swinburne  and  Ashcroft  chlorine  smelting  process  is  a 
method  for  the  treatment  of  mixed  sulphide  ores  such  as  those  of 
zinc  and  lead. 

The  ore,  consisting  of  sulphides  of  lead,  zinc,  iron,  and  manganese, 
with  some  silver,  is  decomposed  by  the  action  of  dry  chlorine  at  a 
temperature  of  600°  C.,  or  700°  C.,  in  a  special  vessel  called  a 
transformer,  forming  a  fused  mixture  of  chlorides  of  the  metals. 
The  sulphur  comes  off  in  the  free  state,  and  can  be  condensed  and 
saved,  while  the  earthy  matter  or  gangue,  from  the  ore,  remains 
suspended  in  the  fused  chlorides.  Enough  heat  is  produced  by  the 
reaction  to  keep  the  transformer  at  the  right  temperature,  which 
can  be  regulated  by  passing  the  chlorine  more  or  less  rapidly.  When 

1  W.  Borchers,  "Electric  Smelting  and  Refining,"  1897,  p.  313. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES    383 

the  vessel  is  full  of  chlorides  they  are  tapped  out,  leaving  enough 
behind  to  serve  as  a  molten  bath  into  which  more  ore  can  be  charged, 
and  through  which  the  chlorine  can  be  passed.  The  molten  chlorides 
are  treated  with  lead,  which  serves  to  remove  the  silver,  and  with 
zinc  to  remove  the  lead.  The  remaining  chlorides  are  dissolved  in 
water,  separated  from  the  gangue  by  filtration,  and  the  iron  and 
manganese  precipitated  chemically  by  the  addition  of  chlorine  and 
zinc  oxide,  leaving  a  solution  of  zinc  chloride  only.  This  solution  is 
evaporated,  and  then  fused  and  electrolyzed  in  a  furnace  shown 
in  outline  in  Fig.  22.  The  products  are  molten  zinc,  which  is 
tapped  off  at  intervals,  and  chlorine,  which  is  compressed  and 
used  again  for  the  treatment  of  fresh  quantities  of  ore.  The 
process  is  one  of  great  interest,  and  is  applicable  to  very  many 
complex  ores  which  are  difficult  to  treat  by  other  methods.  It. 
is  self-contained,  and  does  not  require  any  expensive  reagents,  as 
the  chlorine  for  the  transformer  is  produced  in  the  electrolysis  of 
the  zinc  chloride,  but  the  operations  are  somewhat  complicated, 
and  would  need  very  careful  attention.  At  present  the  only  com- 
mercial installation  is  at  a  plant  of  the  Castner-Kellner  Co.,  which 
has  a  supply  of  chlorine  from  other  processes,  and  uses  it  for  the 
treatment  of  complex  ores  as  described  above,  but  omits  the  final 
electrolysis,  obtaining  the  zinc  in  the  form  of  chloride.  Accounts 
.of  this  process  can  be  found  in  the  "Electrochemical  Industry," 
vol.  i,  p.  412;  vol.  ii,  p.  404;  vol,  iii.  p.  63.  the  transactions  of  the 
Institution  of  Mining  and  Metallurgy  for  1901,  and  the  Mineral 
Industry,  vols.  x  and  xi. 

Aluminium. — This  is  the  most  important  metal  that  is  pro- 
duced solely  in  the  electric  furnace.  Originally  it  was  obtained  by 
complicated  chemical  methods  involving  the  use  of  metallic  sodium 
as  a  reducing  agent,  but  the  electrical  method,  mentioned  in  Chapter 
I,  entirely  supplanted  the  older  processes.  The  common  metals — 
iron,  copper,  lead,  tin,  zinc,  etc. — occur  in  their  ores  as  oxides, 
or  can  easily  be  converted  into  oxides  by  a  roasting  operation, 
and  these  oxides  are  readily  reduced  to  the  metallic  state  by  the 
action  of  carbon  in  an  ordinary  furnace,  because,  at  such  tempera- 
tures, oxygen  has  a  greater  affinity  for  carbon  than  it  has  for  the 
metal.  Other  metals,  however,  such  as  aluminium,  calcium, 
and  sodium,  have  a  greater  affinity  for  oxygen  than  those  already 
mentioned,  and  it  is  very  difficult,  and  in  some  cases  impossible, 
to  reduce  the  oxides  of  these  metals  by  means  of  carbon  at  ordin- 
ary furnace  temperatures.  With  the  aid  of  electricity,  however, 


384  THE  ELECTRIC  FURNACE 

any  metal  can  be  reduced,  either  by  heating  the  oxide  to  a  very 
high  temperature,  at  which  the  affinity  between  the  metal  and 
oxygen  is  lessened,  so  that  the  latter  can  be  removed  by  means 
of  carbon,  or  by  dissolving  the  oxide  or  other  ore  of  the  metal 
in  a  suitable  solvent,  and  applying  an  electrical  force  to  tear  the 
compound  into  two  parts  by  electrolysis,  thus  liberating  the  metal. 
Aluminium,  calcium,  and  other  metals  can  be  reduced  by  carbon 
at  the  high  temperature  of  the  electric  furnace,  but  immediately 
combine  with  a  further  quantity  of  carbon,  forming  carbides.  It 
is,  therefore,  necessary,  when  the  pure  metal  is  desired,  to  employ 
electrolysis  instead  of  the  direct  reduction  with  carbon. 

Aluminium  has  been  termed  "Silver  from  Clay"  as  it  forms 
some  15  or  20  per  cent,  of  ordinary  clay,  but  the  expense  of  ex- 
tracting aluminium  from  clay  would  be  so  great  that  the  richer 
ore,  bauxite,  is  always  employed  as  a  source  of  this  metal.  Bauxite 
consists  of  alumina,  the  oxide  of  aluminium,  combined  with  some 
water  and  associated  with  silica,  oxide  of  iron,  etc.  If  the  natural 
bauxite  were  merely  calcined  to  remove  the  water  and  then  elec- 
trolyzed  in  the  electric  furnace,  the  iron  and  silicon  would  be  re- 
duced more  readily  than  the  aluminium.  The  resulting  metal 
would  therefore  be  impure  and  would  be  almost  useless  for  most  of 
the  purposes  to  which  aluminium  is  applied,  though  an  impure 
metal,  obtained  in  this  way,  would  serve  for  the  production  of  high 
temperatures  by  Dr.  Goldschmidt's  Thermit  process.  For  the 
production  of  the  pure  metal  the  bauxite  must  be  purified  before 
being  introduced  into  the  electrolytic  furnace.  One  method  for 
effecting  this  is  to  digest  the  calcined  bauxite  in  a  solution  of  caustic 
soda,  thus  dissolving  the  alumina  which  is  subsequently  precipitated 
from  the  solution.  A  more  recent  process,  that  of  Hall,  consists  in 
mixing  the  calcined  bauxite  with  a  sufficient  proportion  of  carbon 
or  aluminium  to  reduce  the  whole  of  the  impurities  to  the  metallic 
state.  The  mixture  is  then  charged  into  a  carbon-lined  electric 
furnace  and  melted.  The  iron,  silicon  and  other  impurities  are 
reduced  to  the  metallic  state  and  collect  at  the  bottom  of  the  melted 
mass,  leaving  a  pure  fused  alumina  suitable  for  use  in  the  electrolytic 
furnace. 

The  production  of  aluminium  from  alumina  is  effected  by  elec- 
trolysis as  described  in  outline  in  Chapter  I;  Fig.  I541  represents 
diagrammatically  a  furnace  used  by  the  Pittsburg  Reduction  Com- 

1  From  a  drawing  by  Prof.  J.  W.  Richards  in  the  Journal  of  the  Franklin 
Institute,  May,  1896. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES     385 

pany.  The  furnace  consists  of  an  iron  casing,  B,  thickly  lined 
with  carbon,  D,  and  containing  the  fused  electrolyte,  C,  and  alumin- 
ium, A.  A  number  of  electrodes  of  specially  pure  carbon,  E  E, 
form  the  anode,  while  the  carbon  lining,  D,  and  aluminium,  A, 
form  the  cathode  of  the  furnace.  The  carbon  lining  is  very  thick, 
thus  reducing  the  loss  of  heat,  and  is  provided  with  a  sump  for 
holding  the  aluminium  when  it  is  formed.  •  A  tapping  hole  and 
spout,  leading  from  this  sump,  are  also  provided  though  not  shown 
in  the  figure.  The  electrolyte  consists  of  alumina  dissolved  in  the 
fluorides  of  sodium,  aluminium,  and  calcium.  The  fluorides  are 
not  decomposed  but  merely  serve  as  a  solvent  for  the  alumina. 
Electrolysis  yields  aluminium  at  the  cathode  and  oxygen  at  the 


FIG.  154.  —  Aluminium  furnace. 

anode.     The  oxygen  combines  with  the  carbon  of  the  anodes  as 
shown  in  this  equation: 


Only  a  small  proportion  of  alumina  can  be  dissolved  by  the 
fluorides,  without  unduly  raising  the  temperature  of  the  furnace, 
but  a  quantity  of  alumina  is  placed  on  the  top  of  the  electrolyte 
and  stirred  in  from  time  to  time  as  required.  It  is  desirable  that 
the  furnace  should  be  worked  at  a  low  temperature  and  with  a  low 
current  density,  as  a  high  temperature  causes  the  deposited  metal 
to  dissolve  again  in  the  electrolyte,  and  a  high  current  density 
decomposes  the  fluorides,  liberating  sodium  and  fluorine.  The 
melting  temperature  of  the  most  fusible  mixture  of  cryolite,  the 
natural  fluoride  of  aluminium  and  sodium,  and  alumina  has  been 


25 


386  THE  ELECTRIC  FURNACE 

found  to  be  915°  C.,1  but  it  has  been  stated  that  in  recent  practice 
the  aluminium  furnace  is  worked  at  750°  C.  or  800°  C. 

Recent  accounts  of  the  aluminium  industry,2  state  that  in  one  of 
the  European  works  the  pots  are  8  ft.  long,  4  ft.  to  5  ft.  wide,  and  2 
ft.  high.  The  anodes  are  rectangular  blocks  of  carbon  12  to  20  in. 
(square?)  and  10  to  12  in.  high;  eight  to  twelve  being  hung  in  one 
pot.  The  weight  of  each  anode  is  from  66  to  154  Ib;  the  smaller 
sizes  lasting  from  100  to  140  hours,  and  producing  140  to  200  Ib.  of 
aluminium.  The  current  density  at  the  anode  is  about  650  to  750 
amperes  per  square  foot,  amounting  to  8,000  to  20,000  amperes  per 
pot  according  to  size,  at  an  E.M.F.  of  7  to  8  volts.  The  anodes 
are  kept  2.5  or  3  in.  from  the  molten  aluminium  cathode. 

The  electrolyte  is  stated  in  some  cases  to  have  the  composition 
Al2F6.6NaF.3CaF2.  This  bath  has  a  melting-point  of  800°  C- 
850°  C.,  and  is  usually  kept  between  800°  and  900°  C.  Additions 
of  common  salt  lower  the  melting-point  to  700°  C.,  but  the  salt 
cannot  be  added  regularly  on  account  of  its  volatility.  The  amount 
of  alumina  in  the  bath  varies  from  10  to  20  per  cent.  The  normal 
current  efficiency  is  90-95  per  cent.,  giving  an  output  of  460-600 
Ib.  per  kw.  year,  but  in  many  works  it  is  not  over  60-65  Per  cent., 
or  an  output  of  340  to  440  Ib.  per  kw.  year.3 

The  total  production  of  aluminium  during  the  year  1911  was  nearly 
45,000  tons. 

The  production  of  aluminium  in  the  laboratory,  using  a  current 
of  500  amperes,  is  described  by  Prof.  H.  K.  Richardson.4  He  rec- 
ommends a  mixture  of  85  per  cent,  cryolite  and  15  per  cent,  alumina, 
and  a  current  density  of  19  amperes  per  square  inch  of  the  anode  sur- 
face. The  furnace  used  was  14  in.  long,  7  in.  wide,  and  4  in.  high 
inside.  The  E.M.F.  varied  from  6  to  17  volts,  and  the  temperature 
was  about  900°-!, 000°  C.  The  current  efficiency  obtained  was 
about  60-70  per  cent. 

ELECTROLYTIC  REFINING 

In  electrolytic  refining  the  anode  consists  of  the  impure  metal 
which  is  to  be  refined,  the  electrolyte  contains  some  salt  of  this 

1  F.  R.  Pyne,  Trans.  Amer.  Electrochem.  Soc.,  vol.  x,  p.  63. 

2  Mineral  Industry,  vol.  xx,  1911,  p.  24. 

3  These  figures  are  apparently  based  on  an  E.M.F.  of  about  10  volts  per  pot. 

4  H.  K.  Richardson,  "  Some  Observations  on  Laboratory  Production  of  Alu- 
minium," Trans.  Am.  Electrochem.  Soc.,  xix,  1911,  p.  159. 


ELECTROLYSIS  AND  ELECTROLYTIC  PROCESSES     387 

metal  in  a  state  of  solution  or  fusion  and  the  refined  metal  is  depos- 
ited, by  the  electric  current,  on  the  cathode. 

It  is  well  known  that  many  metals  can  be  refined  in  this  manner, 
but  it  is  not  obvious  why  the  electric  current  should  always  deposit 
the  metal  that  we  wish  to  purify,  instead  of  any  of  the  other  metals 
with  which  it  may  be  associated.  Thus  in  refining  copper,  the  anode 
contains  copper,  silver,  gold,  etc.,  and  copper  is  deposited  on  the 
cathode,  while  in  refining  silver,  an  anode  containing  the  same  three 
metals  yields  a  cathode  of  pure  silver. 

The  nature  of  electrolytic  refining  and  the  reason  why  it  is  so 
generally  applicable  may  be  made  clear  by  a  hydraulic  illustration. 

Fig.  155  represents  a  trough,  having  two  dams,  D  and  F,  and  filled 
with  a  liquid  flowing  from  left  to  right.  Before  reaching  the  first  dam 
the  liquid  is  impure,  being  contaminated  by  the  presence  of  one  or 
more  heavier  liquids,  H,  and  one  or  more  lighter  liquids,  L.  The 


"E-— >K -c- 


FIG.  155. — Electrolytic  refining. 

heavy  liquids  are  stopped  by  the  first  dam,  the  light  liquids  are 
stopped  by  the  second  dam,  and  only  the  pure  liquid,  M,  passes  along 
the  trough.  The  action  of  the  apparatus  depends  on  the  difference  in 
density  of  the  mixed  liquids,  holding  back  first  those  which  are  denser 
and  secondly  those  which  are  less  dense  than  the  liquid  to  be  purified. 
Similarly,  in  electrolytic  refining,  advantage  is  taken  of  the  greater 
or  smaller  solubility  of  the  mixed  metals  in  the  electrolyte.  Those 
that  are  less  soluble  are  stopped  at  the  first  dam  (the  surface  of  the 
anode)  and  those  which  are  more  soluble  pass  into  the  electrolyte, 
with  the  metal  to  be  refined,  but  are  not  deposited  on  the  cathode. 
Referring  to  the  figure,  A  is  the  anode  containing  metal  M  with  less 
soluble  metals  H,  and  more  soluble  metals,  L,  and  D  is  its 
dissolving  surface.  E  is  the  electrolyte  containing  metal  M 
and  those  that  are  more  soluble,  L.  C  is  the  cathode  con- 
sisting only  of  metal,  M,  and  F  is  its  depositing  surface  at  which 
the  second  stage  of  the  purification  takes  place.  It  is  obvious 


388  THE  ELECTRIC  FURNACE 

that  the  electrolytic  refining  process,  like  its  hydraulic  analogue, 
depends  on  there  being  a  great  excess  of  the  metal  (or  liquid)  to  be 
purified,  and  it  is  this  excess  of  one  metal  in  the  anode  (and  elec- 
trolyte) that  really  determines  which  metal  shall  be  obtained  in 
the  refining  process.  The  illustration  shows  that  impurities  will 
tend  to  accumulate  in  the  system  until  the  apparatus  may  cease  to 
operate  and  it  also  suggests  that  greater  purity  can  be  obtained  if 
the  flow  of  material  is  slow  so  that  the  liquids  will  not  become  mixed. 
That  is  to  say  if  the  electric  current-density  is  small.  The  difference 
in  hydraulic  head  between  A  and  C  represents  roughly  the  E.M.F. 
of  the  system  causing  the  flow,  while  the  smaller  difference  of  level 
between  A  and  C  when  there  is  no  flow  might  represent  the  small 
E.M.F.  necessary  to  separate  the  metals  from  their  combination  in 
the  anode. 

The  electrolytic  refining  of  metals  in  aqueous  solutions  forms  an 
important  industry.  It  includes  the  refining  of  copper  in  a  copper 
sulphate  solution,  of  silver  in  silver  nitrate,  of  gold  in  gold  chloride, 
and  lead  in  lead  fluosilicate.  Electrolytic  refining  in  fused  anhydrous 
salts  is  hardly  employed  at  all;  the  only  available  example  being  the 
separation  of  lead  from  bismuth  by  electrolyzing  the  alloy  in  an  elec- 
trolyte of  fused  lead  chloride  as  devised  by  Borchers.1  The  method 
may  find  other  applications  such  as  the  refining  of  aluminium  in 
fused  cryolite,  but  on  account  of  the  greater  difficulty  and  expense 
of  operating  at  high  temperatures  it  would  only  be  used  in  cases 
where  aqueous  electrolytes  could  not  be  employed. 

JDr.  W.  Borchers,  "Electric  Smelting  and  Refining,"  1897  Ed.,  p.  340. 


CHAPTER  XV 
FUTURE  DEVELOPMENTS  OF  THE  ELECTRIC  FURNACE 

In  this  concluding  chapter,  an  attempt  may  be  made  to  indicate 
in  what  directions  future  developments  of  the  electric  furnace  may 
be  expected,  and  to  what  extent  this  development  is  likely  to  pro- 
ceed. .  On  account,  however,  of  the  great  changes  that  take  place 
in  the  economic  conditions  of  the  world,  and  of  the  discoveries  and 
improvements  which  are  made  with  increasing  frequency,  our  ex- 
pectations may  as  easily  prove  to  be  too  moderate  as  too  sanguine. 

The  following  questions  may  be  asked: 

1.  How  far  will  the  electric  current  replace  fuel  in  furnaces  for 
the  smelting  and  refining  of  metals? 

2.  What  untouched  fields  of  usefulness  are  waiting  for  the  electric 
furnace? 

3.  What  limits  are  there  to  the  commercial  development  of  the 
electric  furnace? 

Electric-furnace  operations  may  be  roughly  divided  into  two 
classes,  first,  those  which  can  scarcely  be  effected  in  any  other  way, 
and  in  which  electrical  heating  must  always  hold  the  field,  such 
as  the  production  of  calcium  carbide,  carborundum,  and  aluminium. 
Second,  those  in  which  either  fuel  or  electrical  heat  may  be  used 
with  a  fair  measure  of  efficiency,  and  in  which  the  price  of  the  two 
sources  of  heat  must  be  compared,  in  addition  to  the  efficiency  of 
each,  before  deciding  which  to  employ. 

The  relative  prices  of  coal  and  electrical  energy,  and  the  amount 
of  electrical  power  that  will  be  available,  are  considerations  of  the 
first  importance  in  determining  the  future  of  the  electric  furnace. 

Until  a  few  years  ago  electric  power  was  a  wonderful  and  expen- 
sive commodity,  and  the  idea  of  using  it  for  heating  on  a  commer- 
cial scale  was  preposterous.  About  13  tons  of  coal  were  needed 
to  produce  one  electrical  horse-power  for  a  year,  and  this  electrical 
energy  would  furnish  less  heat  than  one  ton  of  the  original  coal. 
Such  a  method  of  using  coal  was  evidently  extremely  wasteful. 
The  greater  efficiency  of  electrical  heating  somewhat  reduces  it 

389 


390  THE  ELECTRIC  FURNACE 

effective  cost  and  this,  together  with  the  smaller  cost  of  electrical 
energy  when  derived  from  water-power,  has  made  it  cheaper  in 
some  cases  to  use  "white  coal"  instead  of  black,  in  the  furnace. 

In  comparing  the  supplies  and  prices  of  coal  and  electrical  energy, 
it  should  be  remembered  that  one  ton  of  good  coal  produces  as 
much  heat  as  1.33  h.p.  years  of  electrical  energy,  but  that  the  ef- 
ficiency of  the  electrical  furnace  is  from  2  to  30  times  as  great  as 
the  efficiency  of  ordinary  metallurgical  furnaces,  so  that  an  elec- 
trical horse-power  year  will  produce  as  much  effective  heat  as 
several  tons  of  coal.  The  figures  for  different  operations  are  given 
in  Chapter  III,  Table  II. 

The  world's  production  of  coal  at  the  present  time  is  about 
one  thousand  million  tons  a  year,  and  is  steadily  increasing.  The 
electric  furnace  draws  its  energy  mainly  from  water-powers.  The 
water-powers  of  the  world  that  have  already  been  utilized  are  very 
small  in  comparison  with  the  present  coal  output,  having  in  all 
only  about  r  per  cent,  of  the  heating  power  of  the  latter. 

In  view  of  the  fact  that  coal  mining  is  a  long-established  industry, 
while  the  electrification  of  water-powers  is  only  of  recent  growth,  it 
is  reasonable  to  suppose  that  the  latter  will  increase  more  quickly 
than  the  former.  In  both  cases  there  are  limits,  however;  the  coal 
mines  will  ultimately  all  be  discovered  and  worked  out  to  a  depth 
at  which  the  cost  becomes  almost  prohibitive,  while  on  the  other 
hand  the  water-powers  will  all  be  developed,  leaving  only  those 
that  are  too  expensive  to  utilize.  When  these  limits  are  reached 
the  coal  supply  will  have  sunk  to  a  small  proportion  of  the  amount 
needed  for  heating  and  power,  but  the  water-powers  will  continue 
to  give  a  steady  supply  of  power  for  all  time  with  only  maintenance 
and  interest  charges. 

The  exhaustion  of  coal  supplies  may  not  be  reached  for  hundreds 
or  thousands  of  years,  but  if  the  development  of  the  mines  proceeds, 
as  at  present,  at  increasing  rates,  their  practical  depletion  may  be 
less  distant  than  now  appears  probable.  In  any  case  it  seems 
likely  that  as  coal  can  only  be  used  once,  while  water-power  is-  con- 
tinually replenished,  the  latter  may  be  expected  ultimately  to 
largely  replace  the  former  for  motive  power  and  to  some  extent 
for  furnace  work. 

The  present  age,  especially  on  this  continent,  is  one  of  the  bar- 
baric use  of  the  mineral  assets  such  as  coal  and  ore.  As  the  popu- 
lation increases  and  the  development  of  mines  is  pushed  to  its 
limit,  the  increasing  scarcity  both  of  the  ore  and  of  the  fuel  to  smelt 


FUTURE  DEVELOPMENTS  391 

it  will  make  it  necessary  to  spend  more  money  in  utilizing  these  to 
the  very  best  advantage,  using  the  coal  with  the  greatest  economy 
and  extracting  every  possible  product  from  the  ore.  It  has  been 
suggested  that  the  present  enormous  production  of  iron  and  steel, 
for  example,  can  only  represent  a  temporary  condition,  that  of  ex- 
tracting the  iron  from  its  ore.  When  most  of  the  iron-ores  have 
been  converted  into  iron  or  steel  our  descendants  will  have  to  be 
content  to  use  over  again  the  metal  so  produced,  merely  making 
good  the  deficiency  caused  by  rusting  and  the  increase  in  population. 
Iron  is,  however,  a  very  plentiful  metal,  forming  perhaps  4  or  5  per 
cent,  of  the  earth's  crust,  and  the  supply  of  coal  will  last  for  a  large 
number  of  years,  but  the  time  must  come  when  it  would  be  extrava- 
gant to  use  coal,  mined  at  great  expense,  for  the  mere  production 
of  heat.  As  coal  becomes  more  scarce  it  will  be  used  for  its  chemical 
properties  of  reducing  iron  and  other  metals  from  their  ores,  while 
the  necessary  heat  would  be  produced  electrically.  At  that  time 
dwellers  in  northern  countries  may  have  to  heat  their  houses  elec- 
trically, or.  if,  on  account  of  the  large  population  at  that  time,  such 
method  of  heating  were  too  expensive,  they  may  have  to  live  under- 
ground during  the  winter. 

In  this  connection  it  may  be  stated  that  the  water-power  at 
present  developed  in  Canada  amounts  to  1,000,000  h.p.1  and  that 
the  total  available  power  in  that  country  has  been  estimated  at  17,- 
000,000  h.p.2  This  would  have  a  heating  power  equal  to  13,000,000 
tons  of  coal  yearly,  and,  in  view  of  the  greater  efficiency  of  electrical 
heating,  it  might  replace  three  times  that  amount  of  coal  if  used 
for  heating. 

In  the  more  immediate  future  there  will  no  doubt  be  a  great 
development  of  electrical  power,  which  may  in  consequence  replace 
coal  to  some  extent  in  furnace  operations  such  as  the  production 
of  steel  and  iron  from  certain  ores,  and  in  certain  localities;  on  the 
other  hand  the  rapidly  increasing  market  for  electrical  power  will 
tend  to  keep  the  price  from  falling,  relatively  to  the  price  of  coal, 
and  it  is  therefore  unlikely  that  coal  and  coke  will  be  at  all  largely 
replaced  for  smelting  purposes  by  the  electric  current  for  many 
years  to  come. 

When  the  possibilities  of  the  electric  furnace  have  been  more 
fully  ascertained  it  is  likely  that  some  large  water-powers  that  are 

1  Report  of  Commission  of  Conservation  of  Water-powers  of  Canada,  Ottawa, 
1911. 

2  Estimate  mentioned  but  not  authorized  by  the  Commission. 


392  THE  ELECTRIC  FURNACE 

situated  conveniently  with  regard  to  metallic  ores  may  be  utilized 
for  their  reduction,  the  electric  plant  being  available  for  other 
purposes  after  the  exhaustion  of  the  ore  supply.  At  the  present 
time  such  a  large  return  can  be  obtained  from  capital  in  Canadian 
industries  that  only  the  most  easily  developed  water-powers  are 
considered.  When  the  country  becomes  more  thickly  settled  and 
when  capital  is  more  abundant,  a  smaller  return  will  be  expected 
and  the  interest  charges  on  permanent  developments  such  as  hydro- 
electric plants  will  be  less;  thus  enabling  powers  to  be  utilized  that 
would  be  too  costly  under  present  conditions. 

With  regard  to  the  probable  future  developments  of  the  electric 
furnace  it  will  be  instructive  to  review  shortly  the  progress  that  has 
already  been  made: 

I.  The  electric  furnace  has  rendered  available  a  range  of  tem- 
perature from  i, 800°  C.  to  about  3,600°  C.,  which  could  not  pre- 
viously be  reached,  or  in  other  words  it  has  doubled  the  available 
range  of  temperatures  above  the  freezing-point. 

II.  In  the  electric  furnace  substances  can  be  heated  to  any  tem- 
perature within  this  increased  range  with  the  complete  exclusion 
of  air  or  furnace  gases;  a  condition  that  is  very  difficult  and  some- 
times impossible  to  attain  with  other  furnaces.     This  feature  of 
the  electric  furnace  has  enabled  it  to  be  used  for  the  production 
of  substances  like  zinc,  phosphorus  and  carbon  bisulphide. 

III.  Electric  furnaces  are  more  efficient  than  fuel-fired  furnaces 
— particularly  at  high  temperatures — and  in  view  of  this,  electrical 
energy,  though  costing  more  than  an  equivalent  amount  of  fuel, 
has  replaced  fuel  economically  for  a  number  of  purposes. 

IV.  The  electrolytic  furnace  enables  a  direct  electric  tension  to 
be  applied  to  break  up  compounds  that  cannot  be  dealt  with  by 
the  ordinary  chemical  reactions  at  high  temperatures. 

The  increased  range  of  temperature  that  is  now  available  has 
resulted  in  a  complete  new  chemistry  of  high  temperatures.  At 
these  temperatures  all  metals  are  reduced  from  their  oxides  by 
carbon,  and  many  of  them  unite  with  more  carbon  to  form  car- 
bides, some  of  which  have  valuable  properties.  Other  compounds 
such  as  silicides  and  borides  have  also  been  obtained  and  studied. 
No  doubt  in  the  future  many  other  compounds  will  be  obtained, 
from  the  elements  silicon,  carbon,  calcium,  oxygen,  and  aluminium, 
which  form  such  a  large  proportion  of  the  earth's  crust,  as  the 
work  that  has  already  been  done  in  this  direction  can  only  be  con- 
sidered to  have  scratched  lightly  in  the  virgin  soil  that  has  been 


FUTURE  DEVELOPMENTS  393 

placed  at  our  disposal.  Counting  in  the  other  elements,  it  will  be 
seen  what  an  immense  field  for  discovery  lies  open  to  those  who 
are  working  with  the  electric  furnace.  Another  power  furnished 
by  the  electric  furnace  is  the  ability  to  separate  and  purify  substances 
by  fractional  distillation  at  these  high  temperatures.  What  could 
formerly  be  done  by  the  chemist  in  the  separation  of  organic  liquids 
by  distillation  in  glass  vessels  can  now  be  effected  in  the  electric 
furnace  in  the  case  of  such  bodies  as  iron,  lime  and  silica,  not  to 
mention  the  more  fusible  metals  such  as  gold  and  silver.  The  re- 
moval by  distillation  in  the  electric  furnace  of  impurities  from  an- 
thracite, during  its  conversion  into  graphite,  is  one  commercial 
example  of  a  process  which  will  no  doubt  be  largely  employed  in 
the  future. 

The  high  temperatures  that  can  be  obtained,  together  with  the 
ease  with  which  air  can  be  excluded,  and  the  high  efficiency  even 
at  high  temperatures,  has  made  it  economical  to  smelt  electrically 
such  metals  as  chromium,  manganese,  tungsten,  titanium,  and  the 
element  silicon,  whose  reduction  had  been  difficult,  expensive,  and 
incomplete  in  ordinary  furnaces.  Other  elements  will,  no  doubt, 
be  added  to  this  list,  and  a  large  number  of  alloys  and  compounds 
of  these  will  certainly  be  discovered. 

The  electrolytic  furnace  has  already  enabled  aluminium,  sodium, 
potassium,  magnesium,  calcium,  barium,  strontium,  and  other 
metals  to  be  obtained  from  their  fused  salts,  together  with  chlorine 
and  other  substances.  Although  most  of  the  ordinary  metals  that 
are  amenable  to  this  treatment  must  have  been  experimented  with 
already,  there  are  no  doubt  many  new  processes  of  this  character 
waiting  to  be  discovered,  and  it  seems  likely  that  a  far  greater  use 
can  be  made  of  the  alkali  and  alkaline  earth  metals  that  have  been 
made  available  in  quantity  by  this  means. 

The  very  high  temperature  of  the  electric  furnace  has  enabled 
it  to  be  used  for  melting  refractory  metals  and  still  more  refractory 
substances  such  as  silica,  lime,  magnesia  and  alumina.  The  possi- 
bility of  fusing  these  substances  in  quantity  will  lead  to  fresh  uses 
of  these  and  other  materials.  The  conversion  of  amorphous  car- 
bon into  graphite  is  an  example  of  a  physical  change  in  an  elementary 
substance  at  a  high  temperature,  that  may  not  soon  be  duplicated, 
though  the  problem  of  its  conversion  into  the  diamond  is  still 
unsolved  commercially. 

One  very  important  use  of  the  electric  furnace  is  for  experimental 
work  in  the  laboratory.  Here  the  item  of  cost  is  not  a  matter  of 


394  THE  ELECTRIC  FURNACE 

great  importance  as  the  operations  are  usually  small  and  occasional. 
The  results  of  such  experimental  work  are  frequently  very  impor- 
tant and  far  reaching.  For  such  purposes  the  electric  furnace  will 
be  increasingly  employed,  and  standard  forms  will  be  devised  for 
heating  substances,  and  carrying  out  reactions  with  the  complete 
absence  of  oxygen,  carbon,  or  other  objectionable  substance. 

In  order  to  carry  out  chemical  reactions  and  physical  changes  of 
all  kinds  it  is  essential  that  the  pressure  as  well  as  the  temperature 
of  the  system  shall  be  under  complete  control.  The  possibilities  for 
obtaining  these  conditions  that  are  offered  by  the  electric  furnace 
are  beginning  to  be  realized.  They  have  already  been  utilized  in 
such  processes  as  the  synthesis  of  ammonia;  vacuum  and  pressure 
furnaces  have  been  constructed  and  employed  for  various  purposes, 
and  a  wide  field  for  future  development  is  offered  at  this  point. 

One  probable  development  of  the  electric  furnace  in  the  near 
future  is  made  possible  by  .the  intermittent  use  that  is  made  of  elec- 
tric power  for  lighting  and  motor  purposes.  When  electric  power 
is  produced  hydraulically,  large  quantities  could  be  sold  for  electric- 
furnace  work  at  moderate  prices  provided  it  were  only  used  between 
certain  hours.  Although  the  smelting  of  ores  could  hardly  be  car- 
ried on  in  this  intermittent  fashion,  there  are  many  purposes  for 
which  electrical  heat  could  be  applied  in  this  way.  One  of  these  has 
been  suggested  by  Richard  Moldenke  in  an  article  entitled  "Elec- 
tric Smelting  for  the  Foundry,"1  in  which  he  suggests  that  foun- 
drymen  should  make  their  own  steel  castings  by  means  of  the  elec- 
tric furnace,  preferably  the  induction  furnace;  that  even  iron  cast- 
ings would  be  made  better  in  this  way  than  in  the  cupola,  and  that 
the  electric  furnace  would  be  ideal  for  brass  melting.  Such  opera- 
tions could,  of  course,  be  conducted  continuously,  or  as  has  been  sug- 
gested above,  intermittently  so  as  to  obtain  the  power  more  cheaply. 

In  conclusion  it  should  be  remembered  that  water-powers  are  not 
the  only  available  source  of  electrical  power  for  furnace  work.  The 
waste  gas  from  the  iron  blast-furnace  can  also  be  employed.  This 
gas,  used  in  large  gas  engines,  will  frequently  furnish  a  considerable 
amount  of  power  in  excess  of  what  is  needed  for  running  the  plant, 
and  this  excess  could  be  used  for  the  electric  smelting  of  steel  or  simi- 
lar purposes.  Prof.  J.  W.  Richards2  has  stated  that  there  is  as  much 
as  1,000,000  h.p.  available  from  this  source  in  the  United  States 
alone. 

1  Electrochemical  and  Metallurgical  Industry,  vol.  v,  1907,  p.  42. 

2  Trans.  Am.  Electrochem.  Soc.,  vol.  iii,  1903,  p.  67. 


FUTURE  DEVELOPMENTS  395 

For  some  electric-furnace  processes  coal  burned  in  steam  boilers 
may  be  used  to  generate  power,  but  a  considerable  saving  can  now 
be  effected  by  the  use  of  coal,  which  need  not  be  of  very  good  quality, 
in  gas  producers  for  running  large  gas  engines;  while  the  extensive 
deposits  of  peat,  which  are  now  being  developed,  may  be  utilized 
in  the  same  way. 

Other  sources  of  electric  power,  which  may  be  used  in  the  future, 
when  the  price  of  coal  is  getting  higher,  are  the  immense  movements 
of  water  known  as  the  tides.  Attempts  have  also  been  made  to  har- 
ness the  ocean  waves,  whose  great  power  is  attested  by  many  rock- 
bound  coasts,  and  although  their  irregularity  renders  them  unsuit- 
able for  electric  lighting  and  other  uses  of  electricity  where  constancy 
is  an  essential  factor,  it  would  seem  possible  that  certain  smelting 
operations  could  be  conducted  in  this  way.  In  certain  parts  of  the 
world  the  wind  blows  with  considerable  force  and  great  regularity, 
and  this  might  be  utilized  for  the  production  of  electric  power  at  a 
moderate  expense. 

The  strides  of  physical  science  in  recent  years  have  been  so  enor- 
mous that  there  seems  to  be  no  limit  to  what  may  ultimately  be 
possible,  and  if  in  the  future  we  are  able,  as  suggested  by  Lord  Kelvin, 
to  draw  endless  supplies  of  power  from  the  ether  itself,  we  can  wait 
with  quiet  minds  the  exhaustion  of  the  coal  supplies  of  the  world. 


INDEX 


Abrasives,  n,  295,  357 
Acetylene,  .10,  307,  308 
Acheson,  E.  G.,  n,  12,  283,  290,  291 
carborundum   and   graphite   fur- 
naces, n,   12,   28,   114,   130, 
137,  140,  148,  169,  291,  297 
carborundum  works,   silicon  fur- 
nace, 279 

graphite,  30,  96,  284-290 
graphite  company,  286,  288,  289 
Achievements  of  electric  furnaces,  392 
Acker,  caustic  soda  process  and  fur- 
nace, 370 
Adjustable  electrode  holder,  139,  167, 

also,  109-112,  114-117 
Air-cooling  of  furnaces,  85,  307,  311, 

349,  352 
nitrates  and  nitric  acid  from  air, 

12,  346-354 
pump,  3 

Aktiebolaget  Elektrometall,  207 
Alloys,  aluminium,  6 

copper,  6,  47,  91,  281 
ferro-,  13,  15,  265,  277,  281 
silicon,  13,  265-281 
Alloy  steels,  14,  257,  266-271 
Alternating  current,    19,   20,  118-126 
Alumina,  ore  of  aluminium,  6,  7,  384- 

386 

refractory  material,  9,  62,  67 
Aluminium,  6,  7,  47,  91,  149,383-386 
alloys,  6 
furnaces,  7,  385 
Hall  and  Heroult  processes,  6,  7, 

383 

heat  from,  384 
production  of,  7,  386 
uses  of,  52,  384 
Alundum,  62,  74,  167,  357 
Alundum  furnace,  358 
American  Electric  Furnace  Company, 

induction  furnaces,  164,  236 
American  fire-brick,  56-57 


Ammeter,  18,  19,  124 

Amorphous    carbon,    88-91,    96,   97 

104,  107,  282-291 
carbon  electrodes,  96,  289 
graphite,  282 
Amperes  (see  also  individual  furnaces), 

18-20,  45,  97,  100,  123,  137 
Analyses  of  American  fire  clay,  57 
chromite,  61 
ferro-alloys,  266-271 
iron-ores,  189,  193,  194,  206,  252, 

254,  256,  259,  262 
pig  iron,  195,  210 
silicon,  280 

slag,  255,  337,  341,  343 
steel,  51,  52,  216,  253,  255,  257 
zinc  ores,  314,  316,  319,  323 
Andreoli,  quotation  from,  4 
Anode,  37,  95,  366-388 
Anthracite,  calorific  power  of,  49 

for  making  graphite,  12,  284,  287 
for  electrodes,  96 
Aquadag,  291 
Arc,  double,  24,  214 

electric,  i,  2,  3,  19,  21,  63,  133 
furnaces,  1-5,  21-24,38,132-134, 

150-154 

furnaces,   direct-heating,    22,    24 
(see  Girod,  H6roult,  Higgins), 
Salgues,  Siemens.and  Willson) 
furnaces,    independent,     21     (see 
Birkeland  and  Eyde,  Moissan, 
Siemens  and  Stassano) 
resistance  of,  19,  133 
single,  24,  225 
three  phase,  21,  252 
voltage  of,  132-135 
Arch  or  roof  of  furnaces,  27,  57,  60, 

164,  214,  220 
Arendal,  Norway,  electric  furnace  at, 

210 

Argon,  346 

Arsem,  vacuum  electric  furnace,  149, 
158 


397 


398 


INDEX 


Ashcroft     and     Swinburne,     chlorine 

smelting  process,  382 
Ashcroft,  sodium  process  and  furnace, 

374 
Atmosphere,  nitrogen  from,   12,  346- 

354 

Austrian  magnesite,  60 
Automatic  adjustment  of  electrodes, 

4,  139-142 
Automatic     voltage     regulator,     118, 

139-141,  172 


B 


Badische    Analin   and    Soda    Fabrik, 

353 

Balance  sheet  of  heat,  52 
Barium,  382 
Barnes,   Dr.   H.  T.,   specific  heat  of 

water,  44 
Barus,  C.,  thermo-electric  pyrometry, 

144 

vapor  pressure  of  zinc,  330 
Battery,  electric,  i,  3,  4 
Bauxite,  ore  of  aluminium,  384 
purification  of,  384 
refractory  material,  62,  66,  357 
Beard  and  Hutton,  heat  insulation,  67 
Becket,  F.  M.,  use  of  silicon  for  re- 
ducing metals,  272 
water-jackets  for  electrodes,  97 
Belleville,  electric  steel  furnace  at,  256, 

258,  260 
Bergamo,  Italy,  zinc  in  electric  furnace 

at,  323 
Bertani,  and  Casaretti,  zinc  in  electric 

furnace,  323 
Bessemer  converter  or  furnace,  15,  58, 

212 

steel,  13,  15,  174,  212 
Birkeland   and   Eyde,   nitric   acid   in 

electric  furnace,  12,  346,  348 
Bisulphide  of  caroon,   production  in 

electric  furnace,  361 
Bituminous  coal,  calorific  power  of,  39, 

49 

use  of  in  electric  furnace,  192 
Blast-furnace,   coke  used  in,    14,  49, 
174,  175,  179 


Blast-furnace  efficiency,  48,  174,  175 

179,  190 

elimination  of  sulphur  and  phos- 
phorus, 253,  254,  255 
gas,  power  from,  54,  394 

Blast  preheating,  48 

Blister  steel,  212 

Blue  powder,  314,  325,  327,  331 

Boiling  temperature  of  carbon,  63,  66, 
149 

Borchers,  Dr.  W.,  9 

electric  furnaces,  27,  28,  151 
"Electric  Smelting  and  Refining," 
4,  5,  6,  28 

Borrowdale  Mines  Cumberland,  graph- 
ite from,  282 

Bottomly    and    Paget    electroquartz 
furnace,  355 

Boudouard,  O.,  temperature  measure- 
ments, 59,  63,  143 

Boys,  C.  V.,  quartz  filaments,  355 

Bradley    and    Lovejoy    process,     12, 

346 
carbide  furnace,  303 

Brass,  47,  91,  394 

electrode  holders,  114,  115,  117 

Bricks,  refractory,  56-84,  92 

Briquettes  of  iron  ore,  251,  258 

Bristol.  W.  H.,  electric  furnace,  25 

British  Coalite  Company,  ferro-silicon, 

275 
Columbia,  cost  of  electric  power 

in,  53 

thermal  unit,  44,  47,  49 
Broken  Hill  ore,  314,  316 
Bronze  aluminium,  6 

electrode  holders,  n,  108,  168 
in    construction    of    electric    fur- 
naces, 214 
Brown,  W.  G.,  steel  from  ore  in  electric 

furnace,  254 

Bullier  carbide  furnace  301 
Bunsen  battery,  4 
Burgess,  G.  K.,  pyrometry,  143,  146 

table  of  temperatures,  149 
Burning  gases  from  electric  furnace,  6, 

10,  178,  182,  183,  188-196 
heat  from  burning  fuel,  39,  46, 
48,49 


INDEX 


399 


Cables,  electric,  17-20 
Calcium,  3,  10,  369,  381 

cyanamide,  13,  308,  354 
hydride,    382 
nitrate,  348-354 
Calcium  carbide,  production  of,  3, 9, 

10,  13,  298,  307 
furnaces,  10,  109,  no,  in,  113, 

167,  298-307 

ingot  furnaces,  10,  298-303 
resistance  furnaces  305-307 
tapping  furnaces,  303-305 
uses  of,  308 
Calculation   of   electrode   dimensions, 

98—108,  223 
electrical  energy  and  charcoal  for 

iron  smelting,  192—195 
furnace  efficiencies,  43-53 
heat  losses,  67-83 

Caledonia    fire-brick,    electrical   resis- 
tivity, 92,  95 
California,    electric    smelting    in,    15, 

189,  203-207 

Callendar,  resistance  pyrometer,  143 
Calorie,  44,  45 
Calorific  power  of  fuels,  and  electrical 

energy,  17-19,  39-52 
Calorimeter,  46,  48 
Canada,  electric  smelting  in,  14,   15, 

176-180,  185,  256-260 
Canadian  iron  ores,  15,  179,  256,  259 
Government,  16,  324 
Government  Commission,  14,  etc. 

(see  Haanel). 

Capacity  of  electric  furnaces  51,  56, 

142,  and  individual  furnaces 

Carbide  of  calcium,  3,  9,  10,  13,  no, 

in,  113,  167,  298-308 
silicon     (see  carborundum),    291, 

etc. 

Carbides,  9-13,  291-308 
Carbon,  amorphous,  12,  64,  88-91,  96, 

104,  282,  283 
blocks,  27,  29,  335,  360 
boiling  point  of,  63,  66,  149 
broken,  6,  n,  25,  26,  140,  156,  157 
calorific  power  of,  49 


Carbon,  crucibles,  4,  8,  64,  151 

electrical  conductivity  of,  88,  91, 

104 
electrodes,  i,  ?,  4,  5,  6,  19,  89,  95- 

108,  203,  287 
furnace  linings,  8,  13,  63,  64,  66, 

266,  278 

granular,  28,  88,  157 
graphitic   (see  also  graphite),   8, 

64,  96,  104,  282 
for  reducing  ores,  175,  178,  190, 

193,  202,   209,   250,   252,  258 
in  iron  and  steel,  51,  52,  173,  174, 

216,  225,  250,  260 
pole  or  point,  i,  2,  5,  63 
pyrometer,  145 
refractory  material,  8,  63,  66 
resistivity  of,  88,  89,  91,  104 
resistors,  6,  11,  25,  26,  140,  155, 

156,  158,  160,  284-297 
retort  carbon,  6,  64,  96 
rods,  2,  19,  28,  88,  297,  355 
thermal  conductivity  of,  74,  104 
tubes,  155,  156,  160 
vapor,  2,  8, 

Carbon  bisulphide,  361,  363 
Carbon  dioxide,  oxidation  of  zinc  by, 

3^7-331 

Carbon  monoxide,  calorific  power  of,  49 
effect  of  on  condensation  of  zinc, 

329-331 
from    electric    furnaces,     6,     10, 

178-211,  305,  317 
reduction  of  iron  ore  by,  178,  190 
reduction  of  zinc  oxide  by,  330 
Carborundum,    10-12,   64,    66,     157, 

169,  291-297 
discovery  of,  10 
fire-sand,  65,  293,  295 
furnace,  n,  12,  28,  38,  113,  130, 
137,  140  148,    169,  292    (see 
Acheson) 
uses,  u,  64,  295 
Carburite,  52 

Carrier  sodium  process  and  furnace,  3  76 
Casaretti   and   Bertani,    electric   zinc 

smelting,  323 

Casting  of  metals,   frontispiece,   237, 
394 


400 


INDEX 


Cast  iron,  47,  173,  250 

Castner    Kellner    Company,    chlorine 

smelting  process,  383 
Castner,  sodium  process  and  furnace, 

372 

Cathode,  2,  366,  387 
Caustic    soda,    Acker's    process    and 

furnace,  370 
electrolysis,  372,  374 
Cavendish,  combination  of  oxygen  and 

nitrogen,  346 

Centigrade  temperature  scale,  43,  47 
Champagne  (Arie*ge)  France,  electric 

zinc  smelting  at,  316 
Channels  for  molten  resistors,  34,  36, 

234,  238,  240,  244,  248 
Chappuis,  thermo-electric  pyrometry, 

143 
Charcoal,  calorific  power  of,  49 

for  reduction  of  ores,   178,   179, 

207,  195 

furnace  lining,  64 
Charging    and    discharging    furnaces, 

20,  142  (and  individual  fur- 
naces) 

Chart  of  electric  furnaces,  38 
Chats   Falls,   cost  of   electric  power, 

from,  53 
Chemical  compounds,  electrolysis  of, 

36,  366-388 
energy,  19,  45,  368,  391 
Chemistry   at    high    temperatures    of 

electric  furnace,  8,  392 
Chenot  process,  250 
Chicago,  electric  steel  furnace  at,  15, 

220 

Chile  saltpeter,  12 
Chlorine,    from   electric    furnace,    37, 

374,  377 
smelting  process,  Swinburne  and 

Ashcroft,  382 
Chrome  bricks  62,  66 
Chromite,  61,  66 
Chromium,  13,  61,  265,  266,  268,   269 

271,272 
Circuit,  electric,  17,  1 8 

breaker,  172 
Circulation  of  furnace  gases,  202,  205, 

206,  209,  210,  211,  264 


Circulation  of  molten  metal  in  furnaces, 

34,  36,  246,  249 
Clarke,  Dr.  F.  W.,  silicon  in  earth's 

crust,  277 
Classification  of  electric  furnaces,  20- 

38 

Clay  (see  fire-clay)  n,  56,  57,  66 
Coal,  calorific  power  of,  39,  42,  49 
supplies  of,  54,  390 
use  of  in  electric  furnace,  192 
Coal  gas,  calorific  power  of,  49 
Cobalt  ores,  electric  smelting  of,  16, 

340 

Coke,  calorific  power  of,  46,  48,  49 
for  cores  of  electric  furnaces,  n, 
19,  25,  88,  130,  140,  169,  293 
for  crucible  steel  furnaces,  40,  42, 

46 

for  iron  blast-furnace,  48,  174,  176 
for  lining  electric  furnaces,  64 
for  reduction  of  ores  in  electric 

furnace,  175,  176,  182 
graphitized,  89 
resistivity  of,  89 

Colby,  induction  steel  furnace,   fron- 
tispiece, 14,  34,  164,  237-240 
Collens,  C.  L.,  uses  of  Acheson  graph- 
ite, 289 
Combination  of  oxygen  and  nitrogen, 

12,  346 

Combustible  furnace  gases,  6,  10,  49, 
178,  188,  190,  211,  259,  305 
Combustion  of  carbon,  48 
Compensator  coils,  242 
Condensation   of    zinc    vapors,    325, 

331  and  309-338 
Conducting-hearth    arc-furnaces,    22, 

24,  225 

tube  furnaces,  160 
Conduction  of  heat,  41,  55,  60,  67-83, 

99-107 
Conductivity,  electric,  of  carbon  and 

graphite,  88,  91,   104 
fire-bricks,  92-95 
metals  and  alloys,  91,  105 
molten  metals,  87,  105,  140,  248 
molten  slags,  87 

Conductors,  electric,  17,  19,  86-95 
Conley,  electric  furnace,  26 


INDEX 


401 


Connection,  electrical  of  furnaces,  121, 

122 

of  transformer,  122,  208 
Consolidated  Nickel  Company,  342 
Construction  of  electric  furnaces,  1 7- 

20,  55-67,  83-91,  108-117 
Consumption  of  electrical  energy  in 

electric  furnaces,  126-132  and 

individual  furnaces 
of  electrodes,  96-98,289,  and  indi- 
vidual furnaces. 

Contact  resistance  (thermal),  79 
Contents,  resisting,  of  electric  furnaces, 

3°~37»  87,88  (and  individual 

furnaces) 
Continuous  electric  current,  ?,  20,  36, 

120,  366 

furnaces  10,  20,  31,  56 
Convection  of  heat,  77-83 
Cooling  of  electrodes  by  water,  4,  5, 

85,    86,  109-117,    151,    160, 

201,  223,  229,  248,  348 
furnaces,  by  air,  85,  307,  311,  349, 

352 
by  water,  33,  85,  153,  158,  164, 

316,  321,  357,  365,  381 
Copper,  alloys,  6,  47,  91,  281 
aluminium  alloys,  6 
construction  of  electric  furnaces, 
105,  108,  114,  117,  177,  185, 
238,  243,  321,  381 
electric  smelting  of,  16,  340 
melting    temperature    and    heat 

required  to  melt,  47,  149 
silicon  alloys,  281 
vaporized,  5 

Cores  of  resistance  furnaces,  n,  19,  21, 
28,   29,  38,  88-91,  130,  140, 
148,  156,  160,  293 
broken  coke,  u,  19,  25,  88,  130, 

140,  169,  293 
carbon  blocks,  27,  29 
carbon  rods,  17,  28,  88,  89,  284, 

287,  290,  296,  355 
construction  of,  293 
multiple,  296 

resistance  of,  28,  88,  89,  91,  104 
retort  carbon,  6 

Cornu  le  Chatelier  pyrometer,  146 
26 


Corundum,  artificial,  357 
Cost  of  calcium  carbide,  10,  307 
electrical  power,  4,  14,  39-42, 
53-54,  141,  150,  189,  217,  341 
35i,  390 
electric  smelting,   150,   196,   197, 

217,  230,  252,  341 
electrodes,  97,  230,  252,  341 
fuel,  39-42,  179,  217,  341,  390 
iron  ore,  189 
nitric  acid  plant,  353 
phosphorus,  361 
pig  iron,  189 
Stassano  furnace,  252 
steel  from  electric  furnace,   217, 

230,  252,  256 
Cote-Pierron  process  and  furnace,  332, 

333 
Cover,  heat  retaining  of  furnaces,  12, 

19,  27,  56,  67 
Cowles,  E.  H.  and  A.  H.,  291 

Electric  Smelting  and  Aluminium 

Company,  281 

electric  zinc  furnace,  16,  310 
furnace,  for  aluminium  alloys,  6, 

29 

Crocker,  F.  B.,  voltage  of  arc,  133 
Crookes,  nitric  acid  in  electric  arc,  12, 

346 
Crucible,  8,  10,  26,  35,  64,  93,  151,  164, 

212 

electric  furnace,  3,  4,  5,  25,  64,, 
154,   157,  159,  379,  383  (see 
Bristol,  Colby,  Girod,  Howe, 
McGill,  Napier,  Siemens) 
steel,  42,  174,  212 
steel  furnace,  40 
Cryolite,  electrolyte  for  aluminium,  7, 

385 

Crystalline  graphite,  282 
Crystolon,  refractory  material,  65 
Current,  alternating,  19,  20,  118,  124 
continuous  or  direct,   2,   20,   36, 

1 20,  366 

density  in  electrodes,  97,  98-108 
density  in  furnaces,  137-139 
electric,  2,  3,  17,  18,  19 
frequency,    123,    164,    171,    202, 
235,  236,  240,  243 


402 


INDEX 


Current,  polyphase,  120 

regulation,  4,  136,  139,  172 

single  phase,  120 

three-phase,    120,   122,   125,    170, 

202,  206,  220,  246,  349 
transformer,   124,   172 
two-phase,  23,  122,  125,  170,  208, 

246 


D 


Davy,  Sir  Humphry,  i 

Definition  of  electric  furnace,  17 

De  Laval  electric  furnaces,  16,  33,  38, 

85,  312 

Delta  connection  of  furnaces,  121 
Demond,    C.    D.,    "Refractoriness   of 

f  fire-clays,"  56 
Seger  cones,  143 
temperature    measurements,    56, 

143 
Denis    L.     B.,     "Water    powers    of 

Canada,"  53 
Deoxidizing  additions  to  steel,  n,  212, 

2l6,    219,    221,   222,    235,  265- 
28l 

Design  of  electric  furnaces,  55—117,  210 
Despretz,  electric  furnace,  3 
Development  of  electric  furnaces,  1-16, 

389-395 

of  electric  iron  smelting,  197 
Diamond,   a    form  of  carbon,  3,   64, 

282 
production  of,  in  electric  furnace, 

3,  8,  9,  282 
Direct  current  arc,  2 
Direct-heating  arc  furnace,  22-24,  38 
(see  Girod,  H6roult,  Higgins, 
Salgues,  Siemens,  Willson) 
Direct  or  continuous  current,   2,   20, 

36,  120,  366 
Direct  production  of  steel  from  the  ore, 

250-264 

Disc  of  flame,  349 
Discovery  of  carborundum,  n,  291 
electric  arc,  i 
electric  battery,  i 
electric  furnace  graphite,  12,  283 
principle  of  electrical  heating,  3 


Dissociation  of  silicon  carbides,  12,  66, 
283,  288,  294,  296 

Disston,  Henry  and  Sons,  Colby  in- 
duction steel  furnace,  frontis- 
piece, 237 

Distillation  of  metals  in  electric  fur- 
nace, 5,  9,  393 
of  zinc,  309-339 

Dolomite,  refractory  material,  61,  214, 
226,  230,  235 

Domnarfvet,  electric  iron-smelting  fur- 
nace at,  15,  139,  197-203,  210 

Duesseldorf,    carborundum   plant  at, 

295 
Dupre,  A.,  explosive  gas  from  ferro- 

silicon,  271 
Dynamo  or  electric  generator,   3,   4, 

17,  18,  20,  120,  170,  172 


Edstrom,  J.  S.,  electrical  extraction  of 
nitrogen  from  the  air,  351 

Efficiency  of  furnaces,  39-53,  390 
calculation  of,  43 
used  for  melting  metals,  40 

Eichhoff,  Prof.,  operation  of  Heroult 
steel  furnace,  217 

Electric  (see  cross  references) 

Electrical  Metallurgical  Company,  fer- 
ro-alloys in  electric  furnaces, 
269 

Electrochemical  and  Metallurgical  In- 
dustry (frequent  references) 

Electrochemical  equivalents,  369 

Electrochemical  Society,  American, 
transactions  of  (frequent  ref- 
erences) 

Electrode  efficiency,  107 

hearth  arc-furnaces,  22,  24,  225- 

?33 
holders,  n,  19,  108-117,  160,  163, 

168,  223,  251 
losses,  of  energy,  98-108 

of    material,    5,    97,    98,    158, 

202,  206,  209,  210,  225,  252, 

362 

materials,  properties  of,  104 
testing,  102 


INDEX 


403 


Electrodes,  i,  2,  4,  5,  !9,  95-'Io8»  287~ 

289  (and  individual  furnaces) 
central,  22,  31 

copper,  5,  105,  348,  366,  368 
dimensions  of,  98-108,  223 
furnaces  for  graphitizing,  287-289 
iron,  33,  34, 105,  245,  248,  351,  353 
lateral,  30 
loss  of  power  caused  by,  107,  96- 

108 
metal,  4,  5,  33,  226-231,  245,  248, 

348,  35i,  353,  366,  370,  372, 

374,  376,  385 
positive,  3,  5 
ring-shaped,  157 
water-cooled,   4,   5,   85,  109-117, 

151,  160,  201,  223,  248,  348, 

35i,  353 
Electrolysis,  2,  6,  17,  20,  24,  36,  120, 

366-388 

of  alumina,  7,  385 
of  caustic  soda,  370,  372,  374 
of  common  salt,  374,  378,  379 
of  magnesium  chloride,  379 
of  zinc  chloride,  383 
Electrolyte,  7,  366-388 
Electrolytic  furnaces,  7,  17,  19,  36-38 

85,  370-386,  392 
refining,  386 
Electro-magnet,  349 
Electro-mechanical  force,  34,  138 
"Electro-Metals"    steel    furnace,    23, 

231 

Electromotive  force,  18 
Electrons,  2 
Electro-quartz,  355 

Electro  thermic  reduction  of  metals,  3, 
6,    9,    13,    29,    50,    339-345 
and  individual  metals 
production  of  zinc,  309-339 
Emery,  artificial,  357 
Energy,  electric  45,  used  in  furnaces, 
126-132,  and  individual  fur- 
naces 

electric,  cost  of,  4,  14,  39-42,  53, 
55,  141,  150,  189,  217,  341, 

35i,  390 

for  iron-smelting,   175,   179,   182, 
190,  193,  195,  202,  206,  209 


Energy  for  zinc-smelting,  313,  314, 323, 

324,  337 

from  steam-power,  54,  389,  395 
from  water-power,  53-54,  390 
from  winds,  tides  or  the  ether,  395 
heating-power  of,  17—19,  39-52 
production  of,  118—123 

Engelhardt,  V.,  operation  of  induction 
furnace,  239 

Envelope,  refractory  and  heat-retain- 
ing, of  electric  furnace,  19,  55 

Equipment,  electrical,  for  furnaces,  18, 
118—126,  170,  202,  205,  208, 
220,  286,  295,  348,  362 

Essential  parts  of  electric  furnace,  19 

Ether  (the),  electric  power  from,  395 

Ethylene,  C2HU,  calorific  power  of,  49 

European  Commission  Report  (see 
Haanel). 

Europe,  electric  smelting  of  iron  and 
steel  in,  13,  14,  15,  180,  182, 
197,  207,  214,  223,  233,  240 
electric  zinc  smelting  in,  16,  312, 
3i3,  3J5,  323,  332,  338 

Evans,  J.  W.,  steel  from  ore  in  electric 
furnace,  16,  256 

Evans-Stansfield  furnace,  258 

Evaporative  heat  unit,  44 

Expansion  and  contraction  of  re- 
fractory materials,  57,  60 

Experimental  electric  furnaces,  25—27, 
150-170 
zinc  furnace,  318 

Experiments  at  Sault  Ste.  Marie 
(see  Haanel) 

Explosive  gases  from  ferro-silicon,  271, 
277 

External  gas-heating  of  furnaces,  83 

Eyde  and  Birkeland,  nitric  acid  from 
air,  in  electric  furnace,  12, 
346,  348 


Fahrenheit  temperature  scale,  43,  47 
Faure,  electric  resistance  furnace,  5 
Ferro-alloy  works,  Girod,  269 
Ferro-alloys,  13,  15,  265-277,  281 
chromium,  13,  268,  269,  271 


404 


INDEX 


Ferro-manganese,  13,  52,  267,  270 
molybdenum,  13,  269,  271 
nickel,  13,  180,  268,  343 
silicon,  13,  52,  167,  267,  269,  270, 

272,  277,  278,  281 
titanium,  13,  268,  271 
tungsten,  13,  269,  271 
vanadium,  269,  271 
Fertilizers  from  electric  furnaces,  12, 

346-354 

Fery,  optical  measurement  of  tempera- 
ture of  arc,  63 
pyrometer,  147 

Fire-clay  and  bricks,  56,  66,  91,  92 
FitzGerald,  F.  A.  J.,  Acheson  furnaces 

and  products,  12,  293 
electric  furnace  design,  3,  26,  27, 

88,  148 

experiment  in  heat  insulation,  80 
refractories,  61,  62 

Fleming,  J.  A.,  value  of  gram-calorie,  44 
Fluxing  materials,  57 
"Forcing"  furnaces,  36 
Forssell  and  FitzGerald,   resistivity  of 

carbon,  89 

Foster  pyrometer,  148 
Foundry,  use  of  electric  furnace  in,  394 
France,  electric  pig  iron,  180 

electric  steel,  13,  51,  214,  223, 

230,  263 

electric  zinc,  315,  332,  338 
ferro-alloys,  266,  269,  274 
miscellaneous  furnaces,  8,  340 
Frequency  of  electric  currents,   123, 
164,  171,  202,  235,  236,  240, 

243 
Frick  electric  reduction  furnace,  203 

induction  furnace,  246 
Fuel  calorific  power  of,  39—50 

oil,  calorific  power  of,  49 
Furnace  (see  cross  references) 
Fused-quartz,  25,  355 

magnesia,  60 

Future  developments  of  electric  fur- 
Dace,  389 


Canister,  refractory  material,  58 


Gas-carbon,  6,  64,  96 

circulation,    202,    205,    206,    209, 

210,  211,  264 

coal-gas,  calorific  power  of,  49 

engines,  48,  53,  54 

heating,     external,     of    furnaces, 

83 

in  or  from  electric  furnaces,  5,  6, 
10,  37,  49,  86,  151-158,  178- 

211,  251-264,  291-345,  346- 
354,  350,  361-365 

poisonous  and  inflammable  from 

ferro-silicon,  271,  277 
General   Electric   Company   Schenec- 

tady,  159 
Generator,  electric,  17, 18,  20, 120, 170- 

172 
Germany,  electric  steel-furnaces  in,  42, 

230,  243 

Gillett,  furnace  temperatures,  148 
Gin,  G.,  ferro-alloys,  266 

steel  furnaces,  34,  36,   130,   137, 

140,  214,  246-249 

Gin  and  Leleux  carbide  furnace,  305 
Girod,    electric    furnaces    and    ferro- 
alloys, 23,  26,  107,  213,  225, 
266,  269 

Glass  in  electric  furnace,  356 
Goldschmidt,   .  Dr.,     Stassano     steel 

furnace,  127,  252 
thermit  process,  384 
Gram-calorie,  44,  45 
Graphite,  3,  8,   64,  88,  91,  96,   104, 

282—291 

Acheson,  12,  30,  285-290 
amorphous,  282 

Company,  Acheson,  286,  288,  289 
crucibles,  64 
crystalline,  282 
electrodes,  96,  97,  104,  107,  109, 

287-290 

from  electric  furnace,  284 
furnace,  284 

packing,  for  electrode-holder,  185 
pyrometer,  145 
refractory   material,    63,    64,    74, 

333 

soft,    unctuous   or   deflocculated, 
290 


INDEX 


405 


Gray,  G.  W.,  analysis  of  ferro-alloys, 
270 

"Gredag,"  290 

Gronwall  furnaces,  15,  34,  197,  231, 
240 

Gysinge,  Sweden,  induction  steel  fur- 
nace at,  235 


H 


Haanel,  Dr.  E.,  14,  15,  53,  179,  251, 

324 
Commission    to    investigate    the 

zinc     resources     of     British 

Columbia,  313 
Electric   shaft   furnace   at   Dom- 

narfvet,  197 
European   Commission,    14,    180, 

182,  214,  233,  240,  252 
Experiments  at  Sault  Ste  Marie, 

15,  176,  180,  185,  203 
"Recent  advances  in  the  construc- 
tion   of    electric    furnaces," 

197,  203,  246 
Haanel-Heroult,  electric  furnace,  183— 

185 
Halcomb    Steel    Company,    Syracuse, 

Heroult  steel -furnace,   218 
Hall  aluminium  process  and  furnace, 

6,384 
Hallstahammar,  electric  zinc-smelting 

at,  313 
Hansen  C.  A.,   electric  steel-furnace, 

162 
Harbison- Walker,  refractory  materials 

57,  58,  60,  61,  62,  92 
Harbord,  F.  W.,  electric  furnace  steel, 

216 

report  on  zinc-smelting,  313 
"The  Metallurgy  of  Steel,"  250 
Hare  R.,  early  vacuum  furnace,  3 
Harker,  J.  A.,  electrical  tube  furnace, 

27,  161 

thermo-electric  pyrometry,  143 
Harmet,  electric  furnace  for  iron  ores, 

182 
Havard,    F.    T.,    "Refractories    and 

Furnaces,"  58 
Heat,  balance  sheet,  52 


Heat  calories,  44,  45 

convection  and  radiation,  27, 77-83 

flow  of,  67-83 

from     burning      fuel      (calorific 
powers),  39-52 

from  electric  current,  17-19,  45 

from  silicon,  280 

insulation,  12,  26,  27,  32,  56,  67 

losses  of,  41,  67,  77 

meaning  of,  43 

production  of  in  electric  furnace, 
19,  20,  24,  45,  55,  86, 126-132 

specific  heat  of  water,  44 

to  melt  metals,  47 

units,  1 8,  44,  45 
Heating,  coils,  25,  154 

frictional,  34 

rate  of,  36,  45,  55,  56,  126 

surface,  30 

Helfenstein  furnaces,  207,  275,  304 
Hematite,  smelted  in  electric  furnace, 

179,  192,  252,  254,  259,  262 
H6roult-Haanel,  electric  furnace,  183- 

185 
Heroult,  Paul  T.,  aluminium  furnace, 

6,385 
electric  ore-smelting  furnaces,  14, 

15,  112,  128,  176-180 
electric  steel-furnaces,  13,  14,  15, 

23,  50,82,  107,  116,  117,  139, 

162,  214-223 
Heroult-on-the-Pitt,  electric  furnace  at, 

15,  189,  203 
Heroult-Turnbull,     electric     furnace, 

185-189 
Hering,  Dr.  Carl,  electrode  dimensions, 

99 

electric  furnace,  34,  249 
heat-losses  from  furnaces,  67,  75 
pinch  effect,  137 
thermal  units,  45,  47 
Hersey,  Dr.  Milton,  170 
Higgins,  A.  C.,  electric  furnace,  357 
Hofman,  H.  O.,  melting  temperatures 
of  clays,  56,  Seger  cones,  143 
Holders,   electrode,    u,    19,    108-117, 

160,  163,  168,  223,  251 
air-cooled,  111-112 
for  carborundum  furnace,  113 


406 


INDEX 


Holders,  lateral,  109,  114-117 
of  He"roult  steel  furnace,  116 
stuffing-box,  116,  185 
terminal,  109,  110-114 
water-cooled,  109,  no,  114,  117, 

160,  163,  168,  185 
Horry  carbide  furnace,  302 
Horse-power,  electrical,  39,  49 
Howe,  Prof.  H.  M.,  electrical  crucible 

furnace,  25,  154 
melting  temperatures  of  cast-iron, 

47 
Hunt,  Dr.  T.  Sterry,  Cowles'  electric 

furnace,  6 
Huntington     and     Siemens,     electric 

furnace,  i,  5 
Husgafvel  process,  250 
Hutton,  Dr.  R.  S.,  cost  of  water-power, 

53 

electrical  furnaces,  27,  153,  160 
ferro-alloys,  265,  269 
heat-insulation,  67 
pressure  furnace,  153 
Hydro-electric  power,  53-54,  390 
Hydrogen,  calorific  power  of,  49 
Hydrolith,  382 


Ibbotson,  E.  C.,  Kjellin  steel  furnace, 

.236 

Ideal  electric  furnace,  55,  190 
Imbert  Zinc  Process,  334 
Imray,  O.,  electric  furnace  graphite, 

284 

Incomplete  combustion,  41,  48,  190 
Independent-arc     furnaces,     21     (see 

Birkeland  and  Eyde,  Laval, 

Moissan,  Siemens,  Stassano) 
Inductance,  electrical,  123,  177,  225 
Induction  furnaces,  15,  34,  137,  214, 

233-246    (see   Colby,    Frick, 

Gronwall,     Kjellin,     Roden- 

hauser,  Snyder) 
Inertia  electrical,  19 
Infusorial  earth,  73 
Ingalls,  W.  R.,  experiments  on  zinc 

smelting,  324 
Ingot  carbide-furnaces,  298-303 


Instruments    for    electrical    measure- 
ments, 20,  124,  170 
Insulation  electrical,  64,  92,  in,  117, 

144,  146,  154 

of  heat,  12,  26,  27,  32,  56,  67 
Intermittent  electric  furnaces,  10,  20, 

56,  141,  312 

International  Acheson  Graphite  Com- 
pany, 286-289 
Iron  and  steel,  173 

alloys,  13,  14,  15,  173,  265-277 
blast-furnace,  48,   173,   174,  190, 

253,  394 
casing  of  electric  furnaces,  77,  85, 

177,  214,  220,  349,  359,  364 
cast,  47,  173,  250 
cost  of,  189 

malleable  or  wrought,  173,  250 
ore,  51,  173-263 
pig,  173-211,  250 
reduction     from     ore,     173-211, 

250-264 
smelting,  electric,  13,  14,  15,  112, 

166,  175-211 
volatilized,  9,  12 
Ischewsky,    B.    Von,    rotary    electric 

furnace,  26 

Italy,  electric  zinc-smelting,  323 
Stassano  steel-furnace,  14,  251 


Jacobs,    C.    B.,    alundum   in    electric 

furnace,  357 
Jet  effect,  138 
Johnson,     W.     Me. A.,     electric    zinc 

furnace,  16,  311,  336 
Joule,  early  use  of  electric  arc,  4 
Judd,  E.  K.,  graphite,  282 


Kanawha  Falls,  works  at,  268 
Kanolt,   C.  W.,  refractory  materials, 

57,  62 
Kaolin,  57 

Kathode  (or  cathode),  2,  366,  387 
Keeney,  R.  M.,  steel-smelting  furnace, 

261 


INDEX 


407 


Keeney,  R.  M.  ferro-alloys,  272 

Keller,  C.  A.,  electric  smelting  of  iron, 

steel,  and  ferro-alloys,  13,  14 

electric  steel-furnaces,  14,  223,  230 

Leleux,  electric  smelting,  181,  274 

ore-smelting  furnace,  14,  129,  135, 

1 80 
Kelvin  Lord,  energy  from  the  ether, 

395 

Kilogram  calorie,  44 
Kilowatt-hour,  39,  45,  49,  54 
Kjellin,  induction  steel-furnace,  13-15, 

34,36,  129,  137,  140,  233-237 
Kortfors,  Heroult  steel-furnace  at,  215 
Kowalski  and  Moscicki,  nitric  acid, 

346 
Kryptol,  26,  157,  291 


Laboratory  experiments,  16,  25,  150, 

324,  393 
furnaces,  150-172 

Lake  Superior  Power  Company,  180 

LamjDen,  A.,  high  temperature  meas- 
urements, 66,  294 
tube-furnace,  155 

Landis,  G.  C.,  phosphorus-furnace,  359 

La    Neo-Metallurgie,     steel-smelting, 
263 

Lanyon  Zinc  Company,  311 

La    Praz,    Heroult    steel-furnace,    51, 
128,  215 

Lathe,  F.  E.,  s.teel  from  ore  in  electric 
furnaces,  254 

Laval  de,  electric  furnace,  33,  140 
electric  zinc-furnace,  312 

Lead-smelting,  344 

zinc  ores,  chlorine-smelting  proc- 
ess, 382 

Leavitt  and  Company,  Girod  furnace, 
230 

Le  Chatelier,  high-temperature  meas- 
urements, 143 

Leffler  A.,  report  of  Trollhattan  fur- 
nace, 210 

Leleux  and  Gin,  carbide  furnace,  305 

Leleux  Keller  and  Co.,  electric  smelting, 
181,  274 


Lenher,  V.,  H.  Moissan,  "The  electric 
furnace,"  trans  by,  8 

Lewes,  F.  B.,  "Acetylene,"  10,  298 

Light,  electric,  i,  2 

powders  for  retaining  heat,  73 

Lime  and  limestone,  refractory  mate- 
rials, 8,  9,  58,  66 
mortar,  57 

Lincoln,  P.  M.,  resistivity  of  carbon,  89 

Lindblad,  electric  arc-furnace,  231 
electric  induction  furnace,  240 
electric  reduction  furnace,  15,  197 

Linings  of  furnaces,  8,  36,  56-66  (see 
also  individual  furnaces) 

Livet,     France,     Keller    ore-smelting 
furnace  at,  181 

Lloyd,  M.  B.,  and  Dupre,  explosive 
gas  from  ferro-silicon,  271 

Load  factor,  54 

Longmuir,  P.,  analysis  of  ferro-alloys 
270 

Louvrier-Louis,  zinc-furnace,  338 

Lovejoy,  nitric  acid  process,  12,  346 

Lucke,  Prof.  C.  E.,  cost  of  water-power, 

53 

Lummer,  temperature  of  arc,  63 
Lyon,   Prof.   D.   A.,   electric   pig-iron 

furnace,  15,  205 


M 


Mabery,  Prof.  C.  F.,  Cowles'  furnace,  6 

reduction  of  metals  by  carbon,  9 
Magnesia  (magnesite)  refractory  mate- 
rial, 8,  59,  66,  74,  92,  95,  220 
Magnesium,  369,  379 
Magnetic  circuit-breaker,  172 

leakage,  238,  240,  242 
Magnetite,  smelted  in  electric  furnace, 
179,  189,  193,  194,  202,  206, 

259 

Malleable  iron,  250 
Manganese,  13,  52,  265-271,  369 
Manures,  nitrate,  12,  346-354 
Materials  of  construction  of  furnaces, 

56,  67,  84,  86,  92,  95,  108 
McGill  University,  Metallurgical  De- 
partment,   electrical    equip- 
ment, 170 


408 


INDEX 


McGill  University,  electric  smelting  ex- 
periments, 254,  258,  324 
furnaces    and    electrode-holders, 
25,  114,  117,  163,  164,  166 

Measurement  of  furnace  temperatures, 

2,  142-149,  158 
Measuring  instruments,  electrical,  20, 

124,  170 
Melted  metals  as  resistors,  19,  34,  130, 

137,  140,  164,  233-249 
Melting  contents  of  furnaces,  31 
metals,  4,  34,  40,  46,  51 
metals,   furnaces  for,   4,   34,   40, 
I55»    157    (see    Colby,    Gin, 
Girod,  Harker,  Hering,  Her- 
oult,  Howe,  Kjellin,  Moissan, 
Siemens.) 
temperatures  of  metals,  47 

of  refractory  materials,  57-67 
Meslans  and  Poulenc  furnace,  151 
Mesure  and  Nouel  pyrometer,  146 
Metallic  heating  coils  in  furnaces,  25, 

9°.  154 
Metallurgical  calorific  power,  48 

Department,    McGill    University 

(see  McGill  University) 
Metals,  melting  temperatures  and  heat 

required  to  melt,  47 
Methane  (marsh  gas),  CH4,  49 
Mho,  thermal,  72 
Miscellaneous  uses  of  electric  furnaces, 

346-365 

Missouri,  fire-clay,  56 
Moissan  Henri,  electric  arc-furnaces, 

3,  8,  9,  126,  133 
researches,  3,  8,  9,  291,  298 

Moissanite,  295 

Moldenke,  R.,  use  of  electric  furnace  in 

foundry,  394 

Molybdenum,  13,  269,  271,  272 
Monox,  363-365 
furnace,  364 
Morse  pyrometer,  147 
Mortar  for  fire-bricks,  58 
Moscicki   and    Kowalsjd,    nitric   acid 

process,  346 

Motor-generator,  120,  170 
Multiple  core  furnace,  297 
Muthmann  crucible,  381 


N 


Napier,  electric  arc  furnace,  3 

National  Carbon  Company,  experi- 
ments on  the  resistivity  of 
carbon,  89 

Natural  gas,  49 

Nelson,  B.  C.,  electric  zinc-smelting, 
16,  325 

Nernst  earths,  for  resistors  of  furnaces, 

1 60 
nitric  oxide  in  electric  arc,  347 

New  Jersey  fire-bricks,  56 

Niagara  Falls,  cost  of  electric  power  at, 

53 
electric  furnace  operations  at,  12, 

118 

Nichrome  heating  coils,  154 
Nickel  heating  coils,  154 

smelting  in  electric  furnace,   16, 

342 
Nitric  acid  and  nitrates  from  the  air, 

12,  346-354 
Nitrogen    in    air,    fixed    by    electric 

furnace,  12,  346 

Noble  Electric  Steel  Company,  205 
Northrup,   Dr.,   calculation  of  pinch 

effect,  138 
Norton    Company,    manufacture    of 

alundum,  357 

Norway,  cost  of  water-power  in,  53,  351 

electric  iron-smelting,  15,  210 

electric  zinc-smelting  in,  313 

nitric  acid  from  the  air,  12,  346 

Notodden,  manufacture  of  nitric  acid 

from  the  air  at,  351 
Nouel  and  Mesure  pyrometer,  146 
Nystrom,    E.,    report   of   Trollhattan 

furnace,  210 


O 


Ohm,  electrical,  18,  45 

thermal,  72 
Ohm's  law,  73 
Oil  (fuel),  49 
Oildag,  290 

Ontario,  electric  iron-smelting  in,  15, 
176,  179,  256,  259 


INDEX 


409 


Ontario,  water-power  in,  53 
Open-hearth  furnace,  2,  15,  40,  42,  50, 
57,  60,  134,  174,  212,  214,  218 

steel,  13,  15,  174 

Operation,  of  electric  furnaces,   118- 
149  (see  individual  furnaces) 

regular,  of  furnaces,  42 
Optical  pyrometry,  146-149 
Ores,  aluminium,  6,  7,  384 

copper,  339-342 

iron,  14,  51,  173-263 

lead,  344 

nickel,  342-344 

tin,  344 

zinc,  16,  309-325,  332-338 
Origin  of  electric  furnaces,  i 
Orton  Mine,  259 
Osborne,  C.  G.,  220 
Output  of,  aluminium,  7,  383 

alundum,  359 

calcium  carbide,  10,  308 

carbon  bisulphide,  362 

carborundum,  295 

ferro-alloys,  268,  274 

graphite,  284,  288 

iron,  179,  189,  208,  210 

nitric  acid,  353 

phosphorus,  361 

sodium,  373 

steel,  218,  230,  236,  252,  260 


Paget  and  Bottomly  furnace  for  fused 
quartz,  355 

Patsch,  nitric  acid  plant  at,  353 

Patterson,  W.  H.,  and  Hutton,  electric 
tube-furnace,  27,  160 

Pauling,  nitric  acid  furnace,  351 
nitric  acid  process,  12,  346,  351 

Peat,  fuel,  calorific  power  of,  49 
electric  power  from,  384 

Penn  Yan,  carbon  bisulphide  furnace 
at,  361 

Pepys,   W.   H.,   electrical  heating  of 
iron  wire,  3 

Petavel  and  Hutton  electric  pressure- 
furnace,  153 

Phosphorus,  electric  furnace  for,  360 


Phosphorus,  produced  in  electric  fur- 
nace,  3,  359 

in  iron  and  steel,  212,  254 
Pichou,  electric  arc  furnace,  4 
Pig-iron,  from  electric  furnace,  173- 

211 

furnaces,  166,  176-211 
Pinch  effect,  36,  137 
Pittsburg   Reduction   Company,   alu- 
minium furnace,  384 
Platinum,  melted  in  electric  furnace, 

i,5 

wire  furnace,  24,  26,  154 
Pole-pieces,  245 
Polyphase  currents,  120 
Porter,    C.    G.,    steel   from   iron   ore, 

258 
Potassium,    obtained   by   electrolysis, 

379 

Potter,  Dr.  H.  N.,  electric  tube-fur- 
nace, 27 

heat  of  oxidation  of  silicon,  280 
monox,  363 

Poulenc  and  Meslans  furnace,  151 
Pound  calorie,  44 
Power,  electric,  45,  54,  55.  Il8»  I23» 

126,  137,  139,  170 
cost  of,  4,   14,  39-40,   141,   150, 

189,  217,  341,  351,  390 
density  (power-density)  137 
from  blast-furnace  gas,  54,  394 
from  coal,  54,  389,  395 

from  gas-engines,  54,  395 

from  peat,  395 

from  steam,  54,  395 

from  water-power,  53,  118,  391 

from  winds  or  tides,  395 

production  of,  118 

regulation,  139 

used    in    electric    furnaces,    123, 
126-132,   170,  202  (and  indi- 
vidual furnaces) 
Power  factor  of  furnaces,    123,    164, 

202,  235,  236,  240 
Preheating  of  ore  or  charge,  182,  188, 

190,  202,  206,  211,  251,  258, 

329.  353 

Pressure  furnaces,  153,  394 
Price,  E.  F.,  use  of  ferro-silicon,  272 


410 


INDEX 


Priestley,  combination  of  nitrogen  and 

oxygen,  12,  346 
Producer  gas,  49 
Production  of  heat  in  electric  furnaces, 

19,  20,  24,  45,  55,  86,  126-132 
coal,  54,  390 

Prolong  of  zinc  condenser,  325 
Purification  of  bauxite,  384 
Pyne,   F.  R.,  melting  temperature  of 

cryolite,  386 

Pyritic  smelting  of  copper  ores,  342 
Pyrrhotite    ores,    smelted   in    electric 

furnace,  179,  180 
Pyrometry,  43,  142-149 


Quartz,  fused,  in  electric  furnace,  9, 

355 
Quebec,  Canada,  charcoal  for  electric 

iron-smelting  in,  1 79 


Rayleigh,  nitric  acid  in  electric  arc,  346 
Radiation  of  heat,  21,  77 
Reduction  of  ore  to  metal  in  elect- 
ric furnace,  3,  6,  9,  13,   29, 
5°»  339-345  (and  individual 
metals) 

of  ore   to   metal  in   preliminary 

heating,  259,  329 
Refining,  electrolytic,  386 

of  steel  in   electric   furnace,   36, 

213-249,  251-264 
Refractory  materials,  19,  56,  66,  74,  92 

alumina,  9,  62,  66 

bauxite,  62,  66,  357 

carbon,  8,  63,  66 

carborundum,  n,  64,  66,  157,  291 

carborundum-fire-sand,  65,  295 

chromite  and  chrome  bricks,  61, 
66,  92,  95 

dolomite,  61,  214,  226,  230,  235 

expansion  and  contraction  of,  57, 
60 

fire-clay  and  bricks,  58,  66,  91,  92 

graphite,  64,  282 

heat  conductivities,  60,  67 


Refractory  materials, 

lime  and  lime-stone,  8,  9,  58,  66 
magnesia  and  magnesite  bricks,  8, 

p,  59,  66,  74,  92 

melting  temperatures  of,  57,  62,  66 
silica  and  bricks,  57,  66,  74,  92 
siloxicon,  66,  296 
silundum,  65,  296 
Regenerative  steel-furnaces,  40 
Regulation  of  electric  current,  4,  136, 

139,  172 

of  electric  furnaces  and  regulating 
devices,  4,  20,  26,  32,  139-142 
of  voltage,  118,  141,  170 
Report  of  European  Commission  (see 

(Haanel) 
experiments  at  Sault  Ste.  Marie 

(see  Haanel) 

Resistance,  electrical,  2,  17,  19,  36 
furnaces,  3,  6,  15,  20,  24-38,  135, 
154,   246,   284,  305  and  indi- 
vidual furnaces 
pyrometry,  143 
thermal,  67,  72,  75 
thermal  of  contact,  76 
Resisting  contents  of  furnaces,  liquid, 

33-36 

melting,  31-33 
solid,  30-31 
Resistivity,  electrical,    of    amorphous 

carbon,  solid,  89,  91,  104 
broken  coke  or  carbon,  91 
graphite,  solid,  89,  104 
graphitized  coke,  88,  91 
heated  fire-bricks,  91,  92,  95 
melting  contents  of  furnaces,  87 
molten  metals,  87,  105,  140,  248 
molten  slags,  87 
metals  and  alloys,  91,  105 
thermal,  67,  72,  74,  76,  77,  79 
Resistor  tube-furnaces,  160 

zinc-furnace,  335 
Resistors  of  electric  furnaces,  17,  19, 

20,  24,  86 

carbon  rods  or  blocks,  19,  27,  28, 

29,  88,  155,  158,  160,  297,  355 

cores  of  coke  or  broken  carbon,  6, 

n,   25,  30,  87,  88,  92,   157, 

284-297,  362 


INDEX 


411 


Resistors  of  electric  furnaces, 
melting  ores,  etc.,  31,  176 
molten  metals,  34,  87,  233-249 
molten  slags,  33,  87 
wire  coils,  24,  90,  154 
Reverberatory  furnace,  40,  57,  58 
Reynolds,  L.  B.,  electric  zinc-furnace, 

3i7 

Rheostat  for  regulating  electric  fur- 
naces, 26,  170 

Richards,  Prof.  J.  W.,  Acheson  fur- 
naces, 286,  288 
aluminium,  6 
calorific  power,  48 
Castner  process,  372 
furnace  efficiencies,  40,  43 
heat  to  melt  metals,  47 
thermal  conductivity,  74 
Ries,  H.,  clays  and  fire-bricks,  56 
Roberts-Austen,   Sir  W.  C.,  thermo- 
electric pyrometry,  144 
Robertson,  T.  D.,  report  of  Trollhat- 

tan  furnace,  207 

Rochling-Rodenhauser  induction  fur- 
nace, 243-246 

Rodenhauser  furnace,  15,  243-246 
Rods,  carbon,  2,  19,  28,  88,  297,  355 
Roof,  of  furnace,  27,  57,  60,  164,  214, 

220 

Rossi  Auguste,  J.,  ferro- titanium,  268 
Rotary  electric  furnaces,  26,  252,  355 
Ruthenburg,     electric    furnace,     131, 
287 


Salgues,     electric     zinc-furnace     and 
smelting,  32,  315 

Salt    and    sawdust    in    carborundum 

furnace,  n,  291 
electrolysis  of,  374,  378,  379 

San  Francisco,  price  of  pig-iron  in,  189 

Sarpsborg,     Norway,     electric     zinc- 
smelting  at,  313 

Sault  Ste.  Marie,  electric  iron-smelting 
at,  15,  112,  128,  176,  1 80 

Saunders,  L.  E.,  refractory  materials, 

62 

temperature  measurements,   148, 
294 


Schonherr,  nitric  acid  process,  12,  346, 

353 
Scott  connection  of  transformers,  122, 

208 

Scott,  E.  K.,  refractory  materials,  61 
Seaward   Kouglegan,   silicon  furnace, 

279 

Seger  cones,  pyrometer,  143 
Series-arc  furnaces,  23,  214-225 
Sexton,  A.  H.,  refractory  materials,  56 
Shaft  furnaces,  30,  31,  40,  166,  177- 

211,  258,  343,  361 
Sheffield,  ganister  from,  58 

steel  making  at,  212 
Shook,  G.  A.,  radiation  pyrometry,  146 
Shore,  optical  pyrometer,  147 
Short  circuits,  33,  34,  64,  158 
Shunt  for  electrical  measurements,  1 24 
Siemens,  Sir  W.,  electric  arc  furnace,  i, 

4,  21,  22 
Silica  (icfractory  material),  57,  66,  74, 

92,  95 

electric  furnace  for  fusing,  355 
Silicon,    n,    13,    65,    169,    265,    270, 

277-281 
copper,  281 
eisen,  267 

furnace,  29,  167,  168,  280 
Siloxicon,   furnace   and   manufacture, 

12,  28,  66,  296 
uses  of,  66 
Silundum,  65,  296 
Silver  from  clay,  384 
Single-arc  furnaces,  22,  24,  225 
Single-phase  current,  120 
Slag  corrosive,  57,  6 1 

molten,  33, 36, 87, 191, 3 17, 32 1, 336 
Slags,  57,  174,  177,  179,  253,  255,  260, 

322,  343 

Smelting,  electric, 
copper,  1 6,  340 
cost,  150,  175,  196,  197,  217,  230, 

252,  34i 

furnaces,   31,   85,    162,  (see  indi- 
vidual metals) 
iron,  15, 112, 166, 175-211 
lead,  344 
nickel,  16,  342 
possibilities  in,  189-197,  389-395 


412 


INDEX 


Smelting,  electric 

steel,  15,  250-264 

tin,  344 

zinc,  1 6,  37,  309-339 
Snyder,    F.    T.,    electric   furnace    for 
liquid  zinc,  321 

electric  zinc-smelting,  319,  324 

furnace  heat-losses,  79 

induction     smelting-furnace,    34, 

140,  319 

Soda  caustic,  Acker  process,  370 
Sodium,  Ashcrof t  process  and  furnace, 

374 

Carrier  process  and  furnace,  376 
Castner  process  and  furnace,  372 
chloride,  electrolysis  of,  374,  378, 

379 

Virginia  Company's  furnace,  379 
Source  of  electric  current,  3,  4,  1 8,  20 

(see  power,  electric) 
South  Chicago,  Heroult  steel-furnace, 

220 

Spiegeleissen,  270 
Squirt  effect,  34 
Stalhane,  O.,  electric  iron-smelting,  15, 

197 

induction  steel-furnace,  240 
steel  refining  furnace,  231 
Stansfield,  Dr.  A.,  crucible  steel  from 

electric  furnace,  42 
electric    power    at    McGill    Uni- 
versity, 170 

electric  steel-smelting,  254,  257 
electric  zinc-smelting,  16,  317,  324 
electrical    resistivity    of    heated 

fire-bricks,  92 

electrode  holders,  114, 115,116, 167 
graphite-carbon  pyrometer,  145 
ideal  electric  furnace,  190 
laboratory  furnaces,  26,  163,  166 
symbol  for  pound-calorie,  45 
Star  silica  brick,  electrical  resistivity, 

92,  95 
or  Y-connection  of  transformers 

or  furnaces,  121,  322 
Starting  electric  furnaces,  87,  178,  257, 

3i7 

Stassano,  Capt.,  electric  steel-furnace, 
14,  15,  21,  127,  134,  163,  251 


Steam  engines  for  electric  power,  54, 395 
Steel,  174,  212,  250 

alloy,  14,  257,  266-271 

Bessemer,  13,  15,  174,  212 

blister,  212 

cost  of,  217,  230,  252,  256 

crucible,  42,  174 

from  titaniferous  and  sulphurous 

ores,  256 
in  electric  furnace  from  metallic 

ingredients,  212-249 
in   electric   furnace,    direct   from 

ore,  15,  250-264 
melted  in  electric  furnace,  i,  34, 

51,  162 
open-hearth,   15,  40,  50.   57,  60, 

174,  212 
recarbonization,  52,  212,  218,  221, 

222,  236 

refining,  15,  36,  52,  212-249 

tool,  212 

Steel  furnaces,  electric-arc,  162,  214- 
233,  251-263 

Colby,  165,  237 

"Electro  Metals,"  231 

Evans-Stansfield,  258 

Frick,  246 

Gin,  246 

Girod,  225 

Gronwall,  231,  240 

Hering,  34,  249 

Heroult,  15,  115,  162,  214,  220 

induction,  164,  233-246 

Keeney,  261 

Keller,  223,  230 

Kjellin,  15,  233 

resistance,  246 

Rochling-Rodenhauser,  15,  243 

Stassano,  16,  251 
Steinhart,  O.  J.,  ferro-alloys,  268 
Strontium,  382 
Stuffing-box  for  electrodes,  116,  163, 

185,  201 
Sulphur  in  blast-furnace  pig-iron,  173, 

175,  253 
electric-furnace  pig-iron,  179,  195, 

210 

electric-furnace  steel,  36,  253,  255, 
256,  262,  264 


INDEX 


413 


Surface  condensation  of  zinc  vapors, 

326 
Sweden,  electric  iron-smelting,  15,  32, 

197,  207,  210 
power,  53 

steel-making,  13,  42,  233 
zinc-smelting,  16,  312 
Swedish  Government,  power  for  iron- 
smelting,  207 

Swinburne    and    Ashcroft,    chlorine- 
smelting  process,  382 
Sykes,  W.,  electric  power  from  steam- 
engines,  54 
Syracuse,  Heroult  steel-furnace  in,  218 


Tapping  carbide-furnaces,  303 
Taylor,     E.     R.,     carbon    bisulphide 

furnace,  361 
Temperature,  definition  of,  43 

high,  of  electric  furnaces,  2,  4,  8, 

ii,  392 

measurements,  142-149,  158,  294 
of  aluminium  furnace,  7,  386 
of  arc,  2,  63 
of  carborundum  furnace,  n,  158, 

294 

of  electric  furnaces,  12,  25,  55,  56 
of  melting  metals,  47 
of  melting  quartz,  2,  57,  356 
of   melting   refractory   materials, 

66 
uniform  of  electric  furnaces,  28, 

297 

Testing-furnaces,  experimental,  150 
Thermal  conductivity,  55,  60,  67-77, 
i          104 
measurements,  18,  39,  43,  45,  49, 

80,  102,  142,  158 
ohm  and  mho,  72 
resistivity,  67,  72,  74,  76,  77,  79 
Thermit,  281,  384 
Thermo-electric  pyrometry,  144 
Thermometers,  43 
Thierry  zinc-furnace,  338 
Tholander  charging-bell,  200 
Thompson,    S.    P.,    "Electricity   and 
Magnetism,"  i 


Thomson    and     FitzGerald,     electric 
furnace,  27,  335 

Joule,  electric  arc,  4 
Thomson  electric  welding,  31 
Three-phase  arc,  21,  252 

current,  120,  122,  125,  170,  201, 

206,  246,  349 

furnace,   24,   120,   122,   201,   210, 

220,   246,   252,   275,  304,  321 

generator,  170,  202 
Thwaite,  B.  H.,  electric  power  from 

blast-furnace  gas,  54 
Thwing,  optical  pyrometer,  148 
Tides,  electric  power  from,  395 
Tilting  furnaces,  frontispiece,  36,  162, 
164,  214,  220,  223,  228,  231, 
238,    244,    355    (see    Colby, 
Bottomly,  "Electro-Metals," 
Girod,  Hering,  He'roult,  Kel- 
ler, Rodenhauser,  Wellman) 
Tin    smelting    in    electric    furnaces, 

344 
Titaniferous  ores,  smelted  in  electric 

furnace,  179,  256,  259 
Titanium,  256,  259,  265,  268,  271 
Tone,  F.  J.,  arc-furnace  for  silicon,  278 
carborundum  furnace,  294 
resistance  furnace  for  silicon,  29 
Tool-steel,  47,  52,  212,  216,  235,  259 
Transformer,    connection,    121,    122, 

208,  233,  322 
electric,  20,  35,  118,  170,  202,  205, 

220 
for  electrical  measurements,  124, 

172 

furnaces    (see    Colby,    Gronwall, 
Kjellin,    Rodenhauser,    Sny- 
der,    Frick     induction     fur- 
naces) 
with  voltage  regulation,  119,  170, 

202,   205 

Trollhattan,  electric  iron-furnace,  15, 

207,  210 

electric  zinc-smelting,  313 
Tube-furnaces,  3,  25,  27,  154, 155,  156, 
1 60,  161,  354  (see  also 
Despretz,  Harker,  Hutton, 
Potter,  Lampen,  Schonherr, 
Tucker) 


414 


INDEX 


Tucker,  S.  A.,  calcium  carbide  furnace 

tests,  306 

electric  tube-furnace,  27,  156 
high  temperature  measurements, 

66,  294 
preparation  of  magnesium,  379 

of  silicon,  168 

Tungsten,  265,  269,  271,  272 
Turin,  Stassano  furnace  at,  251 
Turnbull-Heroult,  electric  ore-smelting 

furnace,  185-  189,  196 
Turnbull,    R.,    manufacture    of    elec- 
trodes, 96 
Two-phase  current,  23,  122,  125,  170, 

208,  246 

furnace,  23,  208,  231,  246 
Tyssedahl,  electric  furnace  at,  210 

U 

United  States,  aluminium,  7,  384 
calcium  carbide,  298 
carborundum   and   graphite,    IT, 

284,  291 

electric-furnace  iron,  189,  203 
electric-furnace  steel,  15,  42,  218, 

220,  237,  262 
electric  power,  53 
electric  zinc,  311,  319,  336 
electrolytic  processes,  67,  370 
ferro-alloys,  268,  278 
miscellaneous    electric    furnaces, 

281,  342,  357,  361,  363 
Units,  electrical,  18,  19,  45,  123,  124 

thermal,  18,  43,  44,  45,  67,  72,  79 
Uses  of  electric  furnace  (see  Table  of 

Contents) 
furnace  products,  (see  individual 

product) 


Vacuum  electric  furnace,  3,  149,  158 

Vanadium,  267,  271,  272 

Vapor  of  carbon,  2,  9^  63,  149,  158 

metals,  5,  283,  288,  298,  393 

phosphorus,  359 

refractory  substances,  9,  66,  363, 

393 

silicon,  65,  278,  294,  296,  363 


Vapor  of  sulphur,  149,  361 

zinc,  309,  325,  326,  330 
Violle,  temperature  of  electric  arc,  63 
Virginia  Electrolytic  Company's  sod- 
ium furnace,  379 

Volta,  discovery  of  electric  battery,  i 
Voltage  determined  by  size  of  furnace, 

136 

of  arc,  133 

of  arc  furnaces,  19,  132,  133 
of   electric  furnaces,  18,  132-136 
(see  also  individual  furnaces) 
of  electric  supply,  18,  118,  170 
of   resistance   furnaces,    36,    135, 
(see  also  individual  furnaces) 
regulation  of,  118,  141,  206 
Voltmeter,  19,  125 
Volts  and  voltage,  18,  19,  118,  124 


W 


Waidner,  C.  L.,  methods  of  pyrometry, 
143,  146 

Waldo,  Dr.  Leonard,  production  of 
electric-furnace  steel,  237 

Walls  of  electric  furnace,  19,  24,  26, 
55.  56,  67,  77,  84,  9?  (see  also 
individual  furnaces) 

Wanner,  optical  pyrometer,  147 

Water-cooling,  electrodes,  4,  5,  34,  85, 
86,   109-117,   151,   160,   201, 
223,  229,  248,  348 
furnaces,   33,  85,   153,   158,   164, 
316,  321,  353,  357,  365,  381 

Water-gas,  49 

Water-power,  14,  53,  118,  391 

Wattmeter,  124 

Watts,  45,  49,  72,  123,  124,  126  (see 
individual  furnaces) 

Waves,  electric  power  from,  395 

Weber,  R.  F.,  melting  temperatures  of 
fire-clays,  56 

Weckbecker,  J.f  electric-furnace  graph- 
ite from  charcoal,  284 

Weedon,  the  electric  arc,  2 

Welland,  Ontario,  electric  iron-smelt- 
ing at,  15 

Wellman,  tilting  open-hearth  furnace, 
134,  214,  218 


INDEX 


415 


Wetherill  process  for  zinc  oxide,  338 
Wheeler,  H.  A.,  melting  temperatures 

of  fire-clays,  56 
White,  A.  V.,  water-powers  of  Canada, 

S3 

White-coal,  390 

Wilbur  mine,  electric  smelting  of  ore 
from,  194 

Willson  Aluminium  Company,  produc- 
tion of  ferro-chromium,  268 

Willson,  calcium  carbide  furnace,  10, 

22,   23,   142,  298 

T.  L.,  discovery  of  calcium  carbide 

9,  298 

Wind,  electric  power  from,  395 
Witherspoon,  R.  A.,  manufacture  of 

calcium  carbide,  304 
Wologdine,  S.,  thermal  conductivity, 

75,  76 

Wood  (fuel),  49 
Wright,    J.,    "Electric    furnaces    and 

their  industrial  applications," 

63 
Wrought-iron,  173,  250 


Y   or   star   connection,    electrical,    of 
furnaces  or  transformers,  121, 
*      322 


Zinc,  37,  47,  309 

Broken-Hill  ore,  314,  316 
carbon    filter    in    zinc-condenser, 

329 

chlorine-smelting  of,  382 
concentration  of  zinc  in  ore-charge 

322,  329 
condensation   of  zinc   vapor,   32, 

309,  325,  327,  331 
on  surface  of  condenser,  326, 

327 
on  floating  particles,  327,  328 


Zinc,  condensation  to  liquid  zinc,  313, 

321,325,328 
to  powder  or  grain,  313,  314, 

325,  327,  33i 

condenser  and  prolong,  309,  325 
continuous  and  intermittent  proc- 
esses, 311,  312,  329 
dilution  of  zinc  vapors,  326,  327, 

329 

distillation,  328 

efficiency  of  zinc-furnaces,  309,314 
electric    power    and    energy    for 

smelting,  313,  314,  323 
electrolysis  of  chloride,  37,  382 
furnaces  electric,  arc,  312,  317 
induction,  319 
resistance,  313,  317 
three-phase,  322 
water-cooled,  315,  321 
(see  C6te-Pierron,  Cowles,  Fitz- 
Gerald,  Johnson,  Laval,  Lou- 
vrier,      Reynolds,      Salgues, 
Snyder     Stansfield,  Thierry, 
Thomson) 

oxidation  by  CO2,  327 
oxide,  310 
powder  and  grain,  313,  314,  325, 

327,  33i 

retorts  or  muffles,  309,  310 
roasting  zinc  ores,  309,  317,  319, 

322 
smelting: — 

at  McGill  University,  324 
electrolytic,  319,  382 
Imbert  process,  334 
in  blast  furnace,  310 
in  electric  furnace,  16,  310-339 
in  retort  furnace,  309 
lead-zinc  ores,  314,  316 
preheated  charges,  329 
reactions,  309,  328 
zinc  dross,  313 
vapor  pressure,  of,  326,  330 
Zirconia    powder,     used    in    Harker 
furnace,  161 


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